first human to time travel

Science | February 2, 2021

An Evolutionary Timeline of Homo Sapiens

Scientists share the findings that helped them pinpoint key moments in the rise of our species

Brian Handwerk

Science Correspondent

The long evolutionary journey that created modern humans began with a single step—or more accurately—with the ability to walk on two legs. One of our earliest-known ancestors, Sahelanthropus , began the slow transition from ape-like movement some six million years ago, but Homo sapiens wouldn’t show up for more than five million years. During that long interim, a menagerie of different human species lived, evolved and died out, intermingling and sometimes interbreeding along the way. As time went on, their bodies changed, as did their brains and their ability to think, as seen in their tools and technologies.

To understand how Homo sapiens eventually evolved from these older lineages of hominins, the group including modern humans and our closest extinct relatives and ancestors, scientists are unearthing ancient bones and stone tools, digging into our genes and recreating the changing environments that helped shape our ancestors’ world and guide their evolution.

These lines of evidence increasingly indicate that H. sapiens originated in Africa, although not necessarily in a single time and place. Instead it seems diverse groups of human ancestors lived in habitable regions around Africa, evolving physically and culturally in relative isolation, until climate driven changes to African landscapes spurred them to intermittently mix and swap everything from genes to tool techniques. Eventually, this process gave rise to the unique genetic makeup of modern humans.

“East Africa was a setting in foment—one conducive to migrations across Africa during the period when Homo sapiens arose,” says Rick Potts , director of the Smithsonian’s Human Origins Program. “It seems to have been an ideal setting for the mixing of genes from migrating populations widely spread across the continent. The implication is that the human genome arose in Africa. Everyone is African, and yet not from any one part of Africa.”

New discoveries are always adding key waypoints to the chart of our human journey. This timeline of Homo sapiens features some of the best evidence documenting how we evolved.

550,000 to 750,000 Years Ago: The Beginning of the Homo sapiens Lineage

Genes, rather than fossils, can help us chart the migrations, movements and evolution of our own species—and those we descended from or interbred with over the ages.

The oldest-recovered DNA of an early human relative comes from Sima de los Huesos , the “Pit of Bones.” At the bottom of a cave in Spain’s Atapuerca Mountains scientists found thousands of teeth and bones from 28 different individuals who somehow ended up collected en masse. In 2016, scientists painstakingly teased out the partial genome from these 430,000-year-old remains to reveal that the humans in the pit are the oldest known Neanderthals , our very successful and most familiar close relatives. Scientists used the molecular clock to estimate how long it took to accumulate the differences between this oldest Neanderthal genome and that of modern humans, and the researchers suggest that a common ancestor lived sometime between 550,000 and 750,000 years ago.

Pinpoint dating isn't the strength of genetic analyses, as the 200,000-year margin of error shows. “In general, estimating ages with genetics is imprecise,” says Joshua Akey, who studies evolution of the human genome at Princeton University. “Genetics is really good at telling us qualitative things about the order of events, and relative time frames.” Before genetics, these divergence dates were estimated by the oldest fossils of various lineages scientists found. In the case of H. sapiens, known remains only date back some 300,000 years, so gene studies have located the divergence far more accurately on our evolutionary timeline than bones alone ever could.

Though our genes clearly show that modern humans, Neanderthals and Denisovans —a mysterious hominin species that left behind substantial traces in our DNA but, so far, only a handful of tooth and bone remains—do share a common ancestor, it’s not apparent who it was. Homo heidelbergensis , a species that existed from 200,000 to 700,000 years ago, is a popular candidate. It appears that the African family tree of this species leads to Homo sapiens while a European branch leads to Homo neanderthalensis and the Denisovans.

More ancient DNA could help provide a clearer picture, but finding it is no sure bet. Unfortunately, the cold, dry and stable conditions best for long-term preservation aren’t common in Africa, and few ancient African human genomes have been sequenced that are older than 10,000 years.

“We currently have no ancient DNA from Africa that even comes near the timeframes of our evolution—a process that is likely to have largely taken place between 800,000 and 300,000 years ago,” says Eleanor Scerri, an archaeological scientist at the Max Planck Institute for the Science of Human History in Germany.

300,000 Years Ago: Fossils Found of Oldest Homo sapiens

As the physical remains of actual ancient people, fossils tell us most about what they were like in life. But bones or teeth are still subject to a significant amount of interpretation. While human remains can survive after hundreds of thousands of years, scientists can’t always make sense of the wide range of morphological features they see to definitively classify the remains as Homo sapiens , or as different species of human relatives.

Fossils often boast a mixture of modern and primitive features, and those don’t evolve uniformly toward our modern anatomy. Instead, certain features seem to change in different places and times, suggesting separate clusters of anatomical evolution would have produced quite different looking people.

No scientists suggest that Homo sapiens first lived in what’s now Morocco, because so much early evidence for our species has been found in both South Africa and East Africa. But fragments of 300,000-year-old skulls, jaws, teeth and other fossils found at Jebel Irhoud , a rich site also home to advanced stone tools, are the oldest Homo sapiens remains yet found.

The remains of five individuals at Jebel Irhoud exhibit traits of a face that looks compellingly modern, mixed with other traits like an elongated brain case reminiscent of more archaic humans. The remains’ presence in the northwestern corner of Africa isn’t evidence of our origin point, but rather of how widely spread humans were across Africa even at this early date.

Other very old fossils often classified as early Homo sapiens come from Florisbad, South Africa (around 260,000 years old), and the Kibish Formation along Ethiopia’s Omo River (around 195,000 years old).

The 160,000-year-old skulls of two adults and a child at Herto, Ethiopia, were classified as the subspecies Homo sapiens idaltu because of slight morphological differences including larger size. But they are otherwise so similar to modern humans that some argue they aren’t a subspecies at all. A skull discovered at Ngaloba, Tanzania, also considered Homo sapiens , represents a 120,000-year-old individual with a mix of archaic traits and more modern aspects like smaller facial features and a further reduced brow.

Debate over the definition of which fossil remains represent modern humans, given these disparities, is common among experts. So much so that some seek to simplify the characterization by considering them part of a single, diverse group.

“The fact of the matter is that all fossils before about 40,000 to 100,000 years ago contain different combinations of so called archaic and modern features. It’s therefore impossible to pick and choose which of the older fossils are members of our lineage or evolutionary dead ends,” Scerri suggests. “The best model is currently one in which they are all early Homo sapiens , as their material culture also indicates.”

As Scerri references, African material culture shows a widespread shift some 300,000 years ago from clunky, handheld stone tools to the more refined blades and projectile points known as Middle Stone Age toolkits.

So when did fossils finally first show fully modern humans with all representative features? It’s not an easy answer. One skull (but only one of several) from Omo Kibish looks much like a modern human at 195,000 years old, while another found in Nigeria’s Iwo Eleru cave, appears very archaic, but is only 13,000 years old . These discrepancies illustrate that the process wasn’t linear, reaching some single point after which all people were modern humans.

300,000 Years Ago: Artifacts Show a Revolution in Tools

Our ancestors used stone tools as long as 3.3 million years ago and by 1.75 million years ago they’d adopted the Acheulean culture , a suite of chunky handaxes and other cutting implements that remained in vogue for nearly 1.5 million years. As recently as 400,000 years ago, thrusting spears used during the hunt of large prey in what is now Germany were state of the art. But they could only be used up close, an obvious and sometimes dangerous limitation.

Even as they acquired the more modern anatomy seen in living humans, the ways our ancestors lived, and the tools they created, changed as well.

Humans took a leap in tool tech with the Middle Stone Age some 300,000 years ago by making those finely crafted tools with flaked points and attaching them to handles and spear shafts to greatly improve hunting prowess. Projectile points like those Potts and colleagues dated to 298,000 to 320,000 years old in southern Kenya were an innovation that suddenly made it possible to kill all manner of elusive or dangerous prey. “It ultimately changed how these earliest sapiens interacted with their ecosystems, and with other people,” says Potts.

Scrapers and awls, which could be used to work animal hides for clothing and to shave wood and other materials, appeared around this time. By at least 90,000 years ago barbed points made of bone— like those discovered at Katanda, Democratic Republic of the Congo —were used to spearfish

As with fossils, tool advancements appear in different places and times, suggesting that distinct groups of people evolved, and possibly later shared, these tool technologies. Those groups may include other humans who are not part of our own lineage.

Last year a collection including sophisticated stone blades was discovered near Chennai, India , and dated to at least 250,000 years ago. The presence of this toolkit in India so soon after modern humans appeared in Africa suggests that other species may have also invented them independently—or that some modern humans spread the technology by leaving Africa earlier than most current thinking suggests.

100,000 to 210,000 Years Ago: Fossils Show Homo sapiens Lived Outside of Africa

Many genetic analyses tracing our roots back to Africa make it clear that Homo sapiens originated on that continent. But it appears that we had a tendency to wander from a much earlier era than scientists had previously suspected.

A jawbone found inside a collapsed cave on the slopes of Mount Carmel, Israel, reveals that modern humans dwelt there, alongside the Mediterranean, some 177,000 to 194,000 years ago. Not only are the jaw and teeth from Misliya Cave unambiguously similar to those seen in modern humans, they were found with sophisticated handaxes and flint tools.

Other finds in the region, including multiple individuals at Qafzeh, Israel, are dated later. They range from 100,000 to 130,000 years ago, suggesting a long presence for humans in the region. At Qafzeh, human remains were found with pieces of red ocher and ocher-stained tools in a site that has been interpreted as the oldest intentional human burial .

Among the limestone cave systems of southern China, more evidence has turned up from between 80,000 and 120,000 years ago. A 100,000-year-old jawbone, complete with a pair of teeth, from Zhirendong retains some archaic traits like a less prominent chin, but otherwise appears so modern that it may represent Homo sapiens . A cave at Daoxian yielded a surprising array of ancient teeth , barely distinguishable from our own, which suggest that Homo sapiens groups were already living very far from Africa from 80,000 to 120,000 years ago.

Even earlier migrations are possible; some believe evidence exists of humans reaching Europe as long as 210,000 years ago. While most early human finds spark some scholarly debate, few reach the level of the Apidima skull fragment, in southern Greece, which may be more than 200,000 years old and might possibly represent the earliest modern human fossil discovered outside of Africa. The site is steeped in controversy , however, with some scholars believing that the badly preserved remains look less those of our own species and more like Neanderthals, whose remains are found just a few feet away in the same cave. Others question the accuracy of the dating analysis undertaken at the site, which is tricky because the fossils have long since fallen out of the geological layers in which they were deposited.

While various groups of humans lived outside of Africa during this era, ultimately, they aren’t part of our own evolutionary story. Genetics can reveal which groups of people were our distant ancestors and which had descendants who eventually died out.

“Of course, there could be multiple out of Africa dispersals,” says Akey. “The question is whether they contributed ancestry to present day individuals and we can say pretty definitely now that they did not.”

50,000 to 60,000 Years Ago: Genes and Climate Reconstructions Show a Migration Out of Africa

All living non-Africans, from Europeans to Australia’s aboriginal people, can trace most of their ancestry to humans who were part of a landmark migration out of Africa beginning some 50,000 to 60,000 years ago , according to numerous genetic studies published in recent years. Reconstructions of climate suggest that lower sea levels created several advantageous periods for humans to leave Africa for the Arabian Peninsula and the Middle East, including one about 55,000 years ago.

“Just by looking at DNA from present day individuals we’ve been able to infer a pretty good outline of human history,” Akey says. “A group dispersed out of Africa maybe 50 to 60 thousand years ago, and then that group traveled around the world and eventually made it to all habitable places of the world.”

While earlier African emigres to the Middle East or China may have interbred with some of the more archaic hominids still living at that time, their lineage appears to have faded out or been overwhelmed by the later migration.

15,000 to 40,000 Years Ago: Genetics and Fossils Show Homo sapiens Became the Only Surviving Human Species

For most of our history on this planet, Homo sapiens have not been the only humans. We coexisted, and as our genes make clear frequently interbred with various hominin species, including some we haven’t yet identified. But they dropped off, one by one, leaving our own species to represent all humanity. On an evolutionary timescale, some of these species vanished only recently.

On the Indonesian island of Flores, fossils evidence a curious and diminutive early human species nicknamed “hobbit.” Homo floresiensis appear to have been living until perhaps 50,000 years ago, but what happened to them is a mystery. They don’t appear to have any close relation to modern humans including the Rampasasa pygmy group, which lives in the same region today.

Neanderthals once stretched across Eurasia from Portugal and the British Isles to Siberia. As Homo sapiens became more prevalent across these areas the Neanderthals faded in their turn, being generally consigned to history by some 40,000 years ago. Some evidence suggests that a few die-hards might have held on in enclaves, like Gibraltar, until perhaps 29,000 years ago. Even today traces of them remain because modern humans carry Neanderthal DNA in their genome .

Our more mysterious cousins, the Denisovans, left behind so few identifiable fossils that scientists aren’t exactly sure what they looked like, or if they might have been more than one species. A recent study of human genomes in Papua New Guinea suggests that humans may have lived with and interbred with Denisovans there as recently as 15,000 years ago, though the claims are controversial. Their genetic legacy is more certain. Many living Asian people inherited perhaps 3 to 5 percent of their DNA from the Denisovans.

Despite the bits of genetic ancestry they contributed to living people, all of our close relatives eventually died out, leaving Homo sapiens as the only human species. Their extinctions add one more intriguing, perhaps unanswerable question to the story of our evolution—why were we the only humans to survive?

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Brian Handwerk | READ MORE

Brian Handwerk is a science correspondent based in Amherst, New Hampshire.

Where Does the Concept of Time Travel Come From?

Time; he's waiting in the wings.

The dream of traveling through time is both ancient and universal. But where did humanity's fascination with time travel begin, and why is the idea so appealing?

The concept of time travel — moving through time the way we move through three-dimensional space — may in fact be hardwired into our perception of time . Linguists have recognized that we are essentially incapable of talking about temporal matters without referencing spatial ones. "In language — any language — no two domains are more intimately linked than space and time," wrote Israeli linguist Guy Deutscher in his 2005 book "The Unfolding of Language." "Even if we are not always aware of it, we invariably speak of time in terms of space, and this reflects the fact that we think of time in terms of space."

Deutscher reminds us that when we plan to meet a friend "around" lunchtime, we are using a metaphor, since lunchtime doesn't have any physical sides. He similarly points out that time can not literally be "long" or "short" like a stick, nor "pass" like a train, or even go "forward" or "backward" any more than it goes sideways, diagonal or down.

Related: Why Does Time Fly When You're Having Fun?

Perhaps because of this connection between space and time, the possibility that time can be experienced in different ways and traveled through has surprisingly early roots. One of the first known examples of time travel appears in the Mahabharata, an ancient Sanskrit epic poem compiled around 400 B.C., Lisa Yaszek, a professor of science fiction studies at the Georgia Institute of Technology in Atlanta, told Live Science 

In the Mahabharata is a story about King Kakudmi, who lived millions of years ago and sought a suitable husband for his beautiful and accomplished daughter, Revati. The two travel to the home of the creator god Brahma to ask for advice. But while in Brahma's plane of existence, they must wait as the god listens to a 20-minute song, after which Brahma explains that time moves differently in the heavens than on Earth. It turned out that "27 chatur-yugas" had passed, or more than 116 million years, according to an online summary , and so everyone Kakudmi and Revati had ever known, including family members and potential suitors, was dead. After this shock, the story closes on a somewhat happy ending in that Revati is betrothed to Balarama, twin brother of the deity Krishna. 

Time is fleeting

To Yaszek, the tale provides an example of what we now call time dilation , in which different observers measure different lengths of time based on their relative frames of reference, a part of Einstein's theory of relativity.

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Such time-slip stories are widespread throughout the world, Yaszek said, citing a Middle Eastern tale from the first century BCE about a Jewish miracle worker who sleeps beneath a newly-planted carob tree and wakes up 70 years later to find it has now matured and borne fruit (carob trees are notorious for how long they take to produce their first harvest). Another instance can be found in an eighth-century Japanese fable about a fisherman named Urashima Tarō who travels to an undersea palace and falls in love with a princess. Tarō finds that, when he returns home, 100 years have passed, according to a translation of the tale published online by the University of South Florida . 

In the early-modern era of the 1700 and 1800s, the sleep-story version of time travel grew more popular, Yaszek said. Examples include the classic tale of Rip Van Winkle, as well as books like Edward Belamy's utopian 1888 novel "Looking Backwards," in which a man wakes up in the year 2000, and the H.G. Wells 1899 novel "The Sleeper Awakes," about a man who slumbers for centuries and wakes to a completely transformed London. 

Related: Science Fiction or Fact: Is Time Travel Possible ?

In other stories from this period, people also start to be able to move backward in time. In Mark Twain’s 1889 satire "A Connecticut Yankee in King Arthur's Court," a blow to the head propels an engineer back to the reign of the legendary British monarch. Objects that can send someone through time begin to appear as well, mainly clocks, such as in Edward Page Mitchell's 1881 story "The Clock that Went Backwards" or Lewis Carrol's 1889 children's fantasy "Sylvie and Bruno," where the characters possess a watch that is a type of time machine . 

The explosion of such stories during this era might come from the fact that people were "beginning to standardize time, and orient themselves to clocks more frequently," Yaszek said. 

Time after time

Wells provided one of the most enduring time-travel plots in his 1895 novella "The Time Machine," which included the innovation of a craft that can move forward and backward through long spans of time. "This is when we’re getting steam engines and trains and the first automobiles," Yaszek said. "I think it’s no surprise that Wells suddenly thinks: 'Hey, maybe we can use a vehicle to travel through time.'"

Because it is such a rich visual icon, many beloved time-travel stories written after this have included a striking time machine, Yaszek said, referencing The Doctor's blue police box — the TARDIS — in the long-running BBC series "Doctor Who," and "Back to the Future"'s silver luxury speedster, the DeLorean . 

More recently, time travel has been used to examine our relationship with the past, Yaszek said, in particular in pieces written by women and people of color. Octavia Butler's 1979 novel "Kindred" about a modern woman who visits her pre-Civil-War ancestors is "a marvelous story that really asks us to rethink black and white relations through history," she said. And a contemporary web series called " Send Me " involves an African-American psychic who can guide people back to antebellum times and witness slavery. 

"I'm really excited about stories like that," Yaszek said. "They help us re-see history from new perspectives."

Time travel has found a home in a wide variety of genres and media, including comedies such as "Groundhog Day" and "Bill and Ted's Excellent Adventure" as well as video games like Nintendo's "The Legend of Zelda: Majora's Mask" and the indie game "Braid." 

Yaszek suggested that this malleability and ubiquity speaks to time travel tales' ability to offer an escape from our normal reality. "They let us imagine that we can break free from the grip of linear time," she said. "And somehow get a new perspective on the human experience, either our own or humanity as a whole, and I think that feels so exciting to us." 

That modern people are often drawn to time-machine stories in particular might reflect the fact that we live in a technological world, she added. Yet time travel's appeal certainly has deeper roots, interwoven into the very fabric of our language and appearing in some of our earliest imaginings. 

"I think it's a way to make sense of the otherwise intangible and inexplicable, because it's hard to grasp time," Yaszek said. "But this is one of the final frontiers, the frontier of time, of life and death. And we're all moving forward, we're all traveling through time."

• If There Were a Time Warp, How Would Physicists Find It?
• Can Animals Tell Time?
• Why Does Time Sometimes Fly When You're NOT Having Fun?

Originally published on Live Science .

Adam Mann is a freelance journalist with over a decade of experience, specializing in astronomy and physics stories. He has a bachelor's degree in astrophysics from UC Berkeley. His work has appeared in the New Yorker, New York Times, National Geographic, Wall Street Journal, Wired, Nature, Science, and many other places. He lives in Oakland, California, where he enjoys riding his bike. 

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Who was the first human? Identifying them is tricky, but it was not our species, Homo sapiens

Science Who was the first human? Identifying them is tricky, but it was not our species, Homo sapiens

Where did we come from?

There's something about human evolution that's inherently intriguing; it stirs an innate curiosity about what came before (and lived and died and bred with) our species, Homo sapiens .

But where in our ancestry does the "human" part of "human evolution" begin?

In other words, how far back in time must we go for our ancestors to not be human and be, instead, an ape walking on two legs? What's needed to qualify as "human"?

Getting to the bottom of this is more complicated than it appears, says Tanya Smith, a human evolutionary biologist at Griffith University.

More than a century ago, scientists began classifying fossils depending on whether they appeared to have looked and acted more in line with humans living today — that's us — than ancient hominins, such as the ape-like  Australopithecus afarensis , nicknamed Lucy , which lived a few million years ago.

"Originally it was things like brain size, tool use — what we thought of as hallmark specialisations of humanity that would be different from earlier Australopithecines," Professor Smith says.

In the years since, though, fossil discoveries overturned some of those assumptions.

So where are we at?

Let's start with the here and now. We, Homo sapiens , are in the human bucket — we define what is human.

We're backed up by the Macquarie Dictionary , which states a "human" is "a human being", which, in turn, is "a member of the human race, Homo sapiens ".

Contrary to the Macquarie Dictionary, though, we're not alone in the historical human bucket.

So, let's take a whistlestop tour of our evolutionary history and see where we end up.

Our closest cousins

Travel back in time a few tens of thousands of years, and there were other two-legged primates that looked a lot like us getting around the planet.

They included our closest cousins Homo neanderthalensis , better known as Neanderthals, and a group some consider a sister lineage of Neanderthals called the Denisovans.

Skeletons show Neanderthals were muscular and a bit shorter than us, but had a bigger brain for their size. The Denisovan portrait is fuzzier — the entire reported suite of Denisovan fossils could be counted on two hands — but they likely resembled Neanderthals .

We don't know a huge amount about Neanderthal behaviour, but what's increasingly clear is they weren't the knuckle-dragging, club-wielding oafs depicted in popular culture.

They made tools and art and engaged in symbolic behaviours , creating objects that had uses beyond consuming food.

"You can find …  teeth that had been pierced potentially for wearing or adorning things, and these are from sites that were really strongly associated with Neanderthals," Professor Smith says.

"So it does seem like some basic abstraction and symbolism was practised, at least by the later Neanderthals."

Whether those behaviours originated within Neanderthal groups or were copied when they came in contact with Homo sapiens , we don't know, Professor Smith says.

"But we do know that this definition that only Homo sapiens made art and only Homo sapiens engaged in what seems like abstraction … is falling away."

Alongside fossils and other archaeological remains, traces of Neanderthals and Denisovans are found today as stretches of DNA in our genome , remnants of interbreeding through the ages — not just with us, but with each other too .

So, Professor Smith says, instead of thinking of our species' evolution as a "family tree" — with branches splitting into two species, then going on to split again or become a dead end — think of it more like a braided river, where multiple water channels diverge, flow for a bit, then come back together.

"It's the idea of genetic information potentially mixing in some populations, then splitting, then later in time mixing again and splitting again."

These ancient encounters prompted some researchers to suggest the three types of human should be considered the same species, Australian National University evolutionary biologist João Teixiera says.

"But the truth is, genetic evidence suggests Neanderthals and Denisovans at least are part of the human family."

What's in a name?

Could the answer to our question be as simple as nomenclature?

The Homo part of our and Neanderthals' Latin name means "human" or "man". Over the decades, more members have been added to the Homo genus, such as Homo floresiensis , perhaps better known as "The Hobbit" , and Homo naledi .

So would the first human be the first Homo?

Well … maybe, La Trobe University archaeologist Andy Herries says.

"If we define something in the genus Homo, then we're defining that it is fundamentally more like us.

"The earliest Homo is the beginning of what it means to be human, in a sense."

And yet, this is not without controversy.

"Strictly speaking, the oldest fossil that has been included in the genus Homo is 2.8 million years old from Ledi-Geraru in Ethiopia ," Professor Herries says.

"But lots of people disagree entirely with that assessment. It's half a mandible."

What about cultural practices, like burying their dead, or symbolic representations?

"The behavioural evidence is spotty," Professor Smith says.

"From that period, they were using tools, but we don't know that they were using fire, and certainly we don't think we're burying their dead or creating symbolic representations of things.

"It's not 'til much later in the record that we would get some of the things that we think of as contemporary behaviours."

The likely "first human", she says, was Homo erectus . These short, stocky humans were a real stayer in human evolutionary history.

Estimates vary, but they're thought to have lived from around 2 million to 100,000 years ago, and were the first humans to walk out of Africa and push into Europe and Asia.

They're credited with abstraction, as evidenced by  an engraved shell some half a million years old .

Neanderthals, H. sapiens and Denisovans are considered by some to have evolved from H. erectus populations in different parts of the world: Neanderthals in Europe, H. sapiens in Africa, and perhaps Denisovans in Asia.

Case closed, right?

Well … even this is tricky, because there's an older Homo than H. erectus .

Meet Homo habilis , or "handy man", named because fossil remains were found near a plethora of stone tools.

It appeared on the scene around 300,000 years before the earliest known H. erectus , and its placement in the Homo genus has been contentious, to say the least.

Some researchers suggest it's ape-like enough that it should be shifted to the more ancient Australopithecines, which would strip it of its human or Homo name.

By this point, we've travelled a couple of million years back in human history. Fossilised remains from around the time and earlier are incredibly rare, and what is unearthed tends to be in bits.

"Rarely do you get a full suite of evidence in a single individual. So you'll get a bit of a skull, and then you'll get a bit of a hand and you'll get a bit of a pelvis and you get a couple teeth, but we don't know how they knit together," Professor Smith says.

"It's only later in the fossil record that we have good, full remains from a single Homo erectus .

"Then you have sites where we know we had multiple individuals living at the same time , but [the fossils and artefacts] don't come with labels when you get them out of the ground."

Plus there's the gradual nature of evolution itself, Professor Herries says.

"There are so many different aspects of what makes us human, but they don't all arrive at the same time — that never happens.

"[Walking upright] usually comes first, then potentially stone tools, and then we get big brains, which should be no real great surprise because stone tools allow you to access a much wider range of resources, and that helps your brain get bigger."

So, while there's no absolute line in history with humans on one side and apes on the other, Professor Herries agrees the first human by contemporary measure was likely a  Homo erectus.

"There was a big evolutionary step that happened at about 2 million years to 1.8 million years, at that switch to Homo erectus , that moves towards more complicated stone tools and behaviour. They were the first global travellers.

"They did a lot of things for the first time."

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A history of time travel: the how, the why and the when of turning back the clock

Pop on Aqua's 'Turn Back Time' and settle in

For most of human history, the world didn’t change very quickly. Until the 1700s, kids could largely expect their lives to be similar to their parents, and that their children would have an experience very similar to their own, too. There were obviously changes in how humans lived over longer stretches of time, but nothing that even different generations could easily observe.

My first introduction to science fiction was Valérian and Laureline. I was ten years old. Every Wednesday there was a magazine called Pilote in France, and there was two pages of Valerian every week. It was the first time I’d seen a girl and a guy in space, agents travelling in time and space. That was amazing.

The past is written. The present? We have to deal with it. But the future is a white page. So I don’t understand why people on this white page are putting all this darkness.

God! Let’s have some color! Let’s have some fun! Let’s at least imagine a better world. Maybe we won’t be able to do it, but we have to try.

The industrial revolution changed all of this. For the first time in human history, the pace of technological change was visible within a human lifespan. 

It is not a coincidence that it was only after science and technological change became a normal part of the human experience, that time travel became something we dreamed of.

Time travel is actually somewhat unique in science fiction. Many core concepts have their origins earlier in history. 

The historical roots of the concept of a 'robot' can be seen in Jewish folklore for example: Golems were anthropomorphic beings sculpted from clay. In Greek mythology, characters would travel to other worlds, and it's no coincidence that The Matrix features a character called Persephone. But time travel is different.

The first real work to envisage travelling in time was The Time Machine by HG Wells, which was published in 1895. 

The book tells the story of a scientist who builds a machine that will take him to the year 802,701 - a world in which ape-like Morlocks are evolutionary descendants of humanity, and have regressed to a primitive lifestyle. 

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The book was a product of its time - both in terms of the science played upon (Charles Darwin had only published Origin of the Species 35 years earlier), and the racist attitudes: it is speculated that the Morlocks were inspired by the Morlachs, a real ethnic group in the Balkans who were often characterised as “primitive”.

Real science

But of course, this was science fiction - what about science fact? The two have always been closely linked, and during the early days it was no different. In 1907, the physicist Hermann Minkowski first argued that Einstein’s Special Relativity could be expressed in geometric terms as a fourth dimension (to add to our known three) - which is exactly how Wells visualised time travel in his work of fiction.

The development of Special and then General Relativity was significant as it provided the theoretical backbone for how time travel could be conceived in scientific terms. In 1949 Kurt Gödel took Einstein’s work and came up with a solution which as a mathematical necessity included what he called “closed timelike curves” - the idea that if you travel far enough, time will loop back around (like how if you keep flying East, you’ll eventually end up back where you started).

In other words, using what became known as the Gödel Metric, it is theoretically possible to travel between any one point in time and space and any other. 

There was just one problem: for Gödel’s theory to be right, the universe would have to be spinning - and scientists don’t believe that it is. So while the maths might make sense, Gödel’s universe does not appear to be the one we’re actually living in. Though he never gave up hope that he might be right: Apparently even on this deathbed, he would ask if anyone has found evidence of a spinning universe. And if he does ever turn out to be right, it means that time travel can happen, and is actually fairly straightforward (well, as far as physics goes anyway).

Since Gödel, scientists have continued to hypothesise about time travel, with perhaps the best known example being tachyons - or particles that move faster than the speed of light (therefore, effectively travelling in time). So far, despite one false alarm at CERN in 2011, there is no evidence that they actually exist.

Chancers and hoaxes

Of course, the lack of real science when it comes to time travel has not stopped some people from claiming to have done it. With the likes of Marty McFly and Doctor Who on the brain, chancers and hoaxers have realised that time travel is immediately a compelling prospect. Here’s a couple of amusing examples.

At the turn of the millennium, when the internet was still in its infancy, forums were captivated by the story of John Titor. Titor claimed he was from the year 2036, and had been sent back in time by the government to obtain an IBM 5100 computer. The thinking appeared to be that by obtaining the computer, the government could find a solution to the UNIX 2038 bug - in which clocks could be reset, Millennium Bug-style, leading to chaos everywhere.

Posting on the 'Time Travel Institute' forums, Titor went into details on how his time machine worked:  It was powered by “two top-spin, dual positive singularities”, and used an X-ray venting system. He also gave a potted history of what humanity could expect: A new American civil war in 2004, and World War III in 2015. He also claimed the “many worlds” interpretation of quantum physics was true, hence why he wasn’t violating the so-called “grandfather paradox”.

Titor claimed he was from the year 2036, and had been sent back in time by the government to obtain an IBM 5100 computer.

Okay, so he probably wasn’t a real time traveller, but in the early days of the internet, when anonymity was more commonplace, he truly captured the imaginations of nerdy early adopters who perhaps, just a little bit, hoped that he might be the real thing.

More recently, in 2013, an Iranian scientist named Ali Razeghi claimed to have invented a time machine of sorts. It was supposedly capable of predicting the next 5-8 years for an individual, with up to 98% accuracy. According to The Telegraph , Razeghi said the invention fits into the size of a standard PC case and “It will not take you into the future, it will bring the future to you”. The idea is that the Iranian government could use it to predict future security threats and military confrontations. So perhaps it is time to check in and see if he managed to predict Donald Trump?

So is this the best we can do? Will we ever manage to crack time travel? Some scientists are still sceptical that it could ever be possible. This includes Stephen Hawking, who proposed the 'Chronology Protection Conjecture' – which is what it sounds like. Essentially, he argues that the laws of physics are as they are to specifically make time travel impossible – on all but “submicroscopic” scales. Essentially, this is to protect how causality works, as if we are suddenly allowed to travel back and kill our grandfathers, it would create massive time paradoxes.

Hawking revealed to Ars Technica in 2012 how he had held a party for time travellers, but only sent out invitations after the date it was held. So did the party support his argument that time travel is impossible? Or did he end up spending the evening in the company of John Titor and Doctor Who?

“I sat there a long time, but no one came”, he said, much to our disappointment.

Huge thanks to Stephen Jorgenson-Murray for walking us through some of the more brain-mangling science for this article.

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Can we time travel? A theoretical physicist provides some answers

Emeritus professor, Physics, Carleton University

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Time travel makes regular appearances in popular culture, with innumerable time travel storylines in movies, television and literature. But it is a surprisingly old idea: one can argue that the Greek tragedy Oedipus Rex , written by Sophocles over 2,500 years ago, is the first time travel story .

But is time travel in fact possible? Given the popularity of the concept, this is a legitimate question. As a theoretical physicist, I find that there are several possible answers to this question, not all of which are contradictory.

The simplest answer is that time travel cannot be possible because if it was, we would already be doing it. One can argue that it is forbidden by the laws of physics, like the second law of thermodynamics or relativity . There are also technical challenges: it might be possible but would involve vast amounts of energy.

There is also the matter of time-travel paradoxes; we can — hypothetically — resolve these if free will is an illusion, if many worlds exist or if the past can only be witnessed but not experienced. Perhaps time travel is impossible simply because time must flow in a linear manner and we have no control over it, or perhaps time is an illusion and time travel is irrelevant.

Laws of physics

Since Albert Einstein’s theory of relativity — which describes the nature of time, space and gravity — is our most profound theory of time, we would like to think that time travel is forbidden by relativity. Unfortunately, one of his colleagues from the Institute for Advanced Study, Kurt Gödel, invented a universe in which time travel was not just possible, but the past and future were inextricably tangled.

We can actually design time machines , but most of these (in principle) successful proposals require negative energy , or negative mass, which does not seem to exist in our universe. If you drop a tennis ball of negative mass, it will fall upwards. This argument is rather unsatisfactory, since it explains why we cannot time travel in practice only by involving another idea — that of negative energy or mass — that we do not really understand.

Mathematical physicist Frank Tipler conceptualized a time machine that does not involve negative mass, but requires more energy than exists in the universe .

Time travel also violates the second law of thermodynamics , which states that entropy or randomness must always increase. Time can only move in one direction — in other words, you cannot unscramble an egg. More specifically, by travelling into the past we are going from now (a high entropy state) into the past, which must have lower entropy.

This argument originated with the English cosmologist Arthur Eddington , and is at best incomplete. Perhaps it stops you travelling into the past, but it says nothing about time travel into the future. In practice, it is just as hard for me to travel to next Thursday as it is to travel to last Thursday.

Resolving paradoxes

There is no doubt that if we could time travel freely, we run into the paradoxes. The best known is the “ grandfather paradox ”: one could hypothetically use a time machine to travel to the past and murder their grandfather before their father’s conception, thereby eliminating the possibility of their own birth. Logically, you cannot both exist and not exist.

Read more: Time travel could be possible, but only with parallel timelines

Kurt Vonnegut’s anti-war novel Slaughterhouse-Five , published in 1969, describes how to evade the grandfather paradox. If free will simply does not exist, it is not possible to kill one’s grandfather in the past, since he was not killed in the past. The novel’s protagonist, Billy Pilgrim, can only travel to other points on his world line (the timeline he exists in), but not to any other point in space-time, so he could not even contemplate killing his grandfather.

The universe in Slaughterhouse-Five is consistent with everything we know. The second law of thermodynamics works perfectly well within it and there is no conflict with relativity. But it is inconsistent with some things we believe in, like free will — you can observe the past, like watching a movie, but you cannot interfere with the actions of people in it.

Could we allow for actual modifications of the past, so that we could go back and murder our grandfather — or Hitler ? There are several multiverse theories that suppose that there are many timelines for different universes. This is also an old idea: in Charles Dickens’ A Christmas Carol , Ebeneezer Scrooge experiences two alternative timelines, one of which leads to a shameful death and the other to happiness.

Time is a river

Roman emperor Marcus Aurelius wrote that:

“ Time is like a river made up of the events which happen , and a violent stream; for as soon as a thing has been seen, it is carried away, and another comes in its place, and this will be carried away too.”

We can imagine that time does flow past every point in the universe, like a river around a rock. But it is difficult to make the idea precise. A flow is a rate of change — the flow of a river is the amount of water that passes a specific length in a given time. Hence if time is a flow, it is at the rate of one second per second, which is not a very useful insight.

Theoretical physicist Stephen Hawking suggested that a “ chronology protection conjecture ” must exist, an as-yet-unknown physical principle that forbids time travel. Hawking’s concept originates from the idea that we cannot know what goes on inside a black hole, because we cannot get information out of it. But this argument is redundant: we cannot time travel because we cannot time travel!

Researchers are investigating a more fundamental theory, where time and space “emerge” from something else. This is referred to as quantum gravity , but unfortunately it does not exist yet.

So is time travel possible? Probably not, but we don’t know for sure!

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According to current physical theory, is it possible for a human being to travel through time?

As several respondents noted, we constantly travel through time--just forward, and all at the same rate. But seriously, time travel is more than mere fantasy, as noted by Gary T. Horowitz, a professor of physics at the University of California at Santa Barbara:

"Perhaps surprisingly, this turns out to be a subtle question. It is not obviously ruled out by our current laws of nature. Recent investigations into this question have provided some evidence that the answer is no, but it has not yet been proven to be impossible."

Even the slight possibility of time travel exerts such fascination that many physicists continue to study not only whether it may be possible but also how one might do it.

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One of the leading researchers in this area is William A. Hiscock, a professor of physics at Montana State University. Here are his thoughts on the matter:

"Is it possible to travel through time? To answer this question, we must be a bit more specific about what we mean by traveling through time. Discounting the everyday progression of time, the question can be divided into two parts: Is it possible, within a short time (less than a human life span), to travel into the distant future? And is it possible to travel into the past?

"Our current understanding of fundamental physics tells us that the answer to the first question is a definite yes, and to the second, maybe.

"The mechanism for traveling into the distant future is to use the time-dilation effect of Special Relativity, which states that a moving clock appears to tick more slowly the closer it approaches the speed of light. This effect, which has been overwhelmingly supported by experimental tests, applies to all types of clocks, including biological aging.

"If one were to depart from the earth in a spaceship that could accelerate continuously at a comfortable one g (an acceleration that would produce a force equal to the gravity at the earth's surface), one would begin to approach the speed of light relative to the earth within about a year. As the ship continued to accelerate, it would come ever closer to the speed of light, and its clocks would appear to run at an ever slower rate relative to the earth. Under such circumstances, a round trip to the center of our galaxy and back to the earth--a distance of some 60,000 light-years--could be completed in only a little more than 40 years of ship time. Upon arriving back at the earth, the astronaut would be only 40 years older, while 60,000 years would have passed on the earth. (Note that there is no 'twin paradox,' because it is unambiguous that the space traveler has felt the constant acceleration for 40 years, while a hypothetical twin left behind on a spaceship circling the earth has not.)

"Such a trip would pose formidable engineering problems: the amount of energy required, even assuming a perfect conversion of mass into energy, is greater than a planetary mass. But nothing in the known laws of physics would prevent such a trip from occurring.

"Time travel into the past, which is what people usually mean by time travel, is a much more uncertain proposition. There are many solutions to Einstein's equations of General Relativity that allow a person to follow a timeline that would result in her (or him) encountering herself--or her grandmother--at an earlier time. The problem is deciding whether these solutions represent situations that could occur in the real universe, or whether they are mere mathematical oddities incompatible with known physics. No experiment or observation has ever indicated that time travel is occurring in our universe. Much work has been done by theoretical physicists in the past decade to try to determine whether, in a universe that is initially without time travel, one can build a time machine--in other words, if it is possible to manipulate matter and the geometry of space-time in such a way as to create new paths that circle back in time.

"How could one build a time machine? The simplest way currently being discussed is to take a wormhole (a tunnel connecting spatially separated regions of space-time) and give one mouth of the wormhole a substantial velocity with respect to the other. Passage through the wormhole would then allow travel to the past.

"Easily said--but where does one obtain a wormhole? Although the theoretical properties of wormholes have been extensively studied over the past decade, little is known about how to form a macroscopic wormhole, large enough for a human or a spaceship to pass through. Some speculative theories of quantum gravity tell us that space-time has a complicated, foamlike structure of wormholes on the smallest scales--10^-33 centimeter, or a billion billion times smaller than an electron. Some physicists believe it may be possible to grab one of these truly microscopic wormholes and enlarge it to usable size, but at present these ideas are all very hypothetical.

"Even if we had a wormhole, would nature allow us to convert it into a time machine? Stephen Hawking has formulated a "Chronology Protection Conjecture," which states that the laws of nature prevent the creation of a time machine. At the moment, however, this is just a conjecture, not proven.

"Theoretical physicists have studied various aspects of physics to determine whether this law or that might protect chronology and forbid the building of a time machine. In all the searching, however, only one bit of physics has been found that might prohibit using a wormhole to travel through time. In 1982, Deborah A. Konkowski of the U.S. Naval Academy and I showed that the energy in the vacuum state of a massless quantized field (such as the photon) would grow without bound as a time machine is being turned on, effectively preventing it from being used. Later studies by Hawking and Kip S. Thorne of Caltech have shown that it is unclear whether the growing energy would change the geometry of space-time rapidly enough to stop the operation of the time machine. Recent work by Tsunefumi Tanaka of Montana State University and myself, along with independent research by David Boulware of the University of Washington, has shown that the energy in the vacuum state of a field having mass (such as the electron) does not grow to unbounded levels; this finding indicates there may be a way to engineer the particle physics to allow a time machine to work.

"Perhaps the biggest surprise of the work of the past decade is that it is not obvious that the laws of physics forbid time travel. It is increasingly clear that the question may not be settled until scientists develop an adequate theory of quantum gravity."

John L. Friedman of the physics department at the University of Wisconsin at Milwaukee has also given this subject a great deal of consideration:

"Special relativity implies that people or clocks at rest (or not accelerating) age more quickly than partners traveling on round-trips in which one changes direction to return to one's partner. In the world's particle accelerators, this prediction is tested daily: Particles traveling in circles at nearly the speed of light decay more slowly than those at rest, and the decay time agrees with theory to the high precision of the measurements.

"Within the framework of Special Relativity, the fact that particles cannot move faster than light prevents one from returning after a high-speed trip to a time earlier than the time of departure. Once gravity is included, however, spacetime is curved, so there are solutions to the equations of General Relativity in which particles can travel in paths that take them back to earlier times. Other features of the geometries that solve the equations of General Relativity include gravitational lenses, gravitational waves and black holes; the dramatic explosion of discoveries in radio and X-ray astronomy during the past two decades has led to the observation of gravitational lenses and gravitational waves, as well as to compelling evidence for giant black holes in the centers of galaxies and stellar-sized black holes that arise from the collapse of dying stars. But there do not appear to be regions of spacetime that allow time travel, raising the fundamental question of what forbids them--or if they really are forbidden.

"A recent surprise is that one can circumvent the 'grandfather paradox,' the idea that it is logically inconsistent for particle paths to loop back to earlier times, because, for example, a granddaughter could go back in time to do away with her grandfather. For several simple physical systems, solutions to the equations of physics exist for any starting condition. In these model systems, something always intervenes to prevent inconsistency analogous to murdering one's grandfather.

"Then why do there seem to be no time machines? Two different answers are consistent with our knowledge. The first is simply that the classical theory has a much broader set of solutions than the correct theory of quantum gravity. It is not implausible that causal structure enters in a fundamental way in quantum gravity and that classical spacetimes with time loops are spurious--in other words, that they do not approximate any states of the complete theory. A second possible answer is provided by recent results that go by the name chronology protection: One supposes that quantum gravity allows microscopic structures that violate causality, and one shows that the character of macroscopic matter forbids the existence of regions with macroscopically large time loops. To create a time machine would require negative energy, and quantum mechanics appears to allow only extremely small regions of negative energy. And the forces needed to create an ordinary-sized region with time loops appear to be extremely large.

"To summarize: It is very likely that the laws of physics rule out macroscopic time machines, but possible that spacetime is filled with microscopic time loops.

Time Travel Probably Isn't Possible—Why Do We Wish It Were?

Time travel exerts an irresistible pull on our scientific and storytelling imagination.

Since H.G. Wells imagined that time was a fourth dimension —and Einstein confirmed it—the idea of time travel has captivated us. More than 50 scientific papers are published on time travel each year, and storytellers continually explore it—from Stephen King’s JFK assassination novel 11/22/63 to the steamy Outlander television series to Woody Allen’s comedy Midnight in Paris . What if we could travel back in time, we wonder, and change history? Assassinate Hitler or marry that high school sweetheart who dumped us? What if we could see what the future has in store?

These are some of the ideas that bestselling author James Gleick explores in his thought-provoking new book, Time Travel: A History. Speaking from his home in New York City, he recalls how Stephen Hawking once sent out invitations to a party that had already taken place ; why the Chinese government has branded time travel as “incorrect” and “frivolous” ; and how the idea of time travel is, ultimately, about our desire to defeat death.

Let’s cut right to the chase: What is time?

Oh, no, you didn’t! [ Laughs. ] In A.D. 400, St. Augustine said—and many people have said the same thing since, either quoting him consciously or unconsciously—“What, then, is time? If no one asks me, I know. If I wish to explain it to one that asks, I know not.” I think that is actually not a quip, but quite profound.

The best way to understand time is to recognize that we actually are very sophisticated about it. Over the past century-plus, we’ve learned a great deal. The physicist John Archibald Wheeler said, “Time is nature’s way to keep everything from happening all at once.” If you look it up in a dictionary, you get stuff like, “The general term for the experience of duration.” But that’s just completely punting because what is duration ?

I try to steer away from aphorisms and dictionary definitions, just to say two things. First, that we have a lot of contradictory ways of talking about time. We think of time as something we waste, spend, or save, as if it’s a quantity. We also think of time as a medium we are passing through every day, a river carrying us along. All of these notions are aspects of a complicated subject that has no bumper sticker answer.

When does the idea of time travel first appear in the West? And how did it impact popular culture?

I assumed, as a person who always read sci-fi a lot when I was a kid, that time travel is an obvious idea we’re born knowing and fantasizing about. And that it must always have been part of human culture, that there must be time travel Greek myths and Chinese legends. But there aren’t! Time travel turns out to be a very new idea that essentially starts with H.G. Wells’s 1895 novel, The Time Machine . Before that nobody thought of putting the words time and travel together. The closest you can come before that is people falling asleep, like Rip Van Winkle, or fantasies like Charles Dickens’s A Christmas Carol .

For Hungry Minds

The beginning of my book is an attempt to answer the question, “Why? Why not before? Why suddenly at the end of the 19 th century was it possible— necessary— for people to dream up this crazy fantasy?” Even though it’s H.G. Wells who does it, people pick up his ball very quickly and run with it. You find it in American science fiction that started appearing in pulp magazines in the 1920s and 1930s, or in the great new modernist literature of Marcel Proust’s In Search of Lost Time , James Joyce, and Virginia Woolf.

All these writers were suddenly making time their explicit subject, twisting time in new ways, inventing new narrative techniques to deal with time, to explore the vagaries of memory or the way our consciousness changes over time.

In 1991, Stephen Hawking wrote a paper called “Chronology Protection Conjecture , ” in which he asked: If time travel is possible, why are we not inundated with tourists from the future? He has a point, doesn’t he?

Yes! He even scheduled a party and sent out an invitation inviting time travelers to come to a party that had taken place in the past. Then he observed that none of them had shown up. [Laughs.] Hawking is one of these physicists who love playing with the idea of time travel. It’s irresistible because it’s so much fun! When he talks about the paradoxes of time travel it’s because he’s reading the same science fiction stories as the rest of us.

The paradoxes started appearing in magazines aimed mostly at young people in the 1920s. Somebody wrote in and said, “Time travel is a weird idea, because what if you go back in time and you kill your grandfather? Then your grandfather never meets your grandmother and you’re never born.” It’s an impossible loop.

Hawking, like other physicists, decided, “Time is my business. What if we take this seriously? Can we express this in physical terms?” I don’t think he succeeded but what he proposed was that the reason these paradoxes can’t happen is because the universe takes care of itself. It can’t happen because it didn’t happen. That’s the simple way of saying what the chronology protection conjecture is.

How have the Internet and other new technologies changed our perception and experience of time?

We are just beginning to see what the Internet is doing to our perception of time. We are living more and more in this networked world in which everything travels at light speed. We are multitasking and experiencing new forms of simultaneity, so the Internet appears to us as a kind of hall of mirrors. It feels as though we’re embedded in an ever expanding present.

Our sense of the past changes because in some ways the past becomes more vivid than ever. We’re looking at the past on our video screens and it’s just as vivid if the movie is about something that happened 20 years ago, as if it is a live stream. We can’t always tell the difference. On the other hand, the past that’s more distant—and isn’t available in video form—starts to seem more remote and fuzzier. Maybe we are forgetting how to visualize the past from reading histories. We’re entering a new period of time confusion, in which we suddenly find ourselves in what looks like an unending present.

In 2011, the Chinese government issued an extraordinary denunciation of the idea of time travel. What was their beef?

They thought it was corrupting and decadent. It’s a reminder that time travel is neither a simple nor innocent idea. It’s very powerful. It enables us to imagine alternative universes, and this is another line that science fiction writers have explored. What if someone was able to go back in time and kill Hitler?

Time travel is also a powerful way of allowing us to imagine what the future might bring. A lot of futurists nowadays tend to be dystopian. Time travel gives us ways of exploring how the worst tendencies of our current societies could grow even worse. That’s what George Orwell did in 1984 . I imagine the Chinese government doesn’t particularly want the equivalent of 1984 to be published in Beijing. [ Laughs. ]

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More than 50 scientific papers a year are now published on the idea of time travel. why are scientists drawn to the subject.

Scientists live in the same science fictional universe as all the rest of us. Time travel is a sexy and romantic idea that appeals to the physicist as much as it appeals to every teenager. I don’t think scientists are ever going to solve the problem of time travel for us but they still love to talk about wormholes and dark matter.

There’s a fascinating coincidence in the early history that when H.G. Wells needed to set the stage for his time machine hurtling into the future, he decided not to just jump right into his story but set the scene with a framing device—his time traveler lecturing a group of friends on the science of time—in order to justify the possibility of a time machine. His lecture introduces the idea that time is nothing more than a fourth dimension, that traveling through time is analogous to traveling through space. Since we have machines that can take us into any of the three special dimensions, including balloons and elevators, why shouldn’t we have a machine able to travel through the fourth dimension?

A decade later, Einstein burst onto the scene with his theory of relativity in which time is a fourth dimension , just like space. Soon after that, Hermann Minkowski pronounced that, henceforth, we were not going to talk about space and time as separate quantities but as a union of the two, spacetime , a four-dimensional continuum in which the future already exists and the past still exists.

I’m not claiming that Einstein read H.G. Wells 10 years before. But there was something in the air that both scientists and imaginative writers were empowered to visualize time in a new way. Today, that’s the way we visualize it. We’re comfortable talking about time as a fourth dimension.

You quote Ursula K. Le Guin , who writes, “Story is our only boat for sailing on the river of time.” Talk about storytelling and its relationship to time.

One of the things that has happened, along with our heightened awareness of time and its possibilities, is that people who invent narratives have learned very clever new techniques. Literal time travel is only one of them. You don’t actually need to send your hero into the future or into the past to write a story that plays with time in clever new ways. Narrative is also how everybody, not just writers, constructs a vision of our own relationship with time. We imagine the future. We remember the past. When we do that, we’re making up stories.

Psychologists are learning something that great storytellers have known for some time, which is that memory is not like computer retrieval. It’s an active process. Every time we remember something we are remembering it a little bit differently. We’re retelling the story to ourselves.

If time travel is impossible, why do we continue to be so fascinated with the idea?

One of the reasons is we want to go back and undo our mistakes. When you ask yourself, “If I had a time machine, what would I do?” sometimes the answer is, “I would go back to this particular day and do that thing over.” I think one of the great time travel movies is Groundhog Day , the Bill Murray movie where he wakes up every morning and has to live the same day over and over again. He gradually realizes that perhaps fate is telling him he needs to do it over, right. Regret is the time traveler’s energy bar. But that’s not the only motivation for time travel. We also have curiosity about the future and interest in our parents and our children. A lot of time travel fiction is a way of asking questions about what our parents were like, or what our children will be like.

At some point during the four years I worked on this book, I also realized that, in one way or another, every time travel story is about death. Death is either explicitly there in the foreground or lurking in the background because time is a bastard, right? Time is brutal. What does time do to us? It kills us. Time travel is our way of flirting with immortality. It’s the closest we’re going to come to it.

This interview was edited for length and clarity.

Simon Worrall curates Book Talk . Follow him on Twitter or at simonworrallauthor.com .

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Time Travel and Modern Physics

Time travel has been a staple of science fiction. With the advent of general relativity it has been entertained by serious physicists. But, especially in the philosophy literature, there have been arguments that time travel is inherently paradoxical. The most famous paradox is the grandfather paradox: you travel back in time and kill your grandfather, thereby preventing your own existence. To avoid inconsistency some circumstance will have to occur which makes you fail in this attempt to kill your grandfather. Doesn’t this require some implausible constraint on otherwise unrelated circumstances? We examine such worries in the context of modern physics.

1. Paradoxes Lost?

2. topology and constraints, 3. the general possibility of time travel in general relativity, 4. two toy models, 5. slightly more realistic models of time travel, 6. the possibility of time travel redux, 7. even if there are constraints, so what, 8. computational models, 9. quantum mechanics to the rescue, 10. conclusions, other internet resources, related entries.

• Supplement: Remarks and Limitations on the Toy Models

Modern physics strips away many aspects of the manifest image of time. Time as it appears in the equations of classical mechanics has no need for a distinguished present moment, for example. Relativity theory leads to even sharper contrasts. It replaces absolute simultaneity, according to which it is possible to unambiguously determine the time order of distant events, with relative simultaneity: extending an “instant of time” throughout space is not unique, but depends on the state of motion of an observer. More dramatically, in general relativity the mathematical properties of time (or better, of spacetime)—its topology and geometry—depend upon how matter is arranged rather than being fixed once and for all. So physics can be, and indeed has to be, formulated without treating time as a universal, fixed background structure. Since general relativity represents gravity through spacetime geometry, the allowed geometries must be as varied as the ways in which matter can be arranged. Alongside geometrical models used to describe the solar system, black holes, and much else, the scope of variation extends to include some exotic structures unlike anything astrophysicists have observed. In particular, there are spacetime geometries with curves that loop back on themselves: closed timelike curves (CTCs), which describe the possible trajectory of an observer who returns exactly back to their earlier state—without any funny business, such as going faster than the speed of light. These geometries satisfy the relevant physical laws, the equations of general relativity, and in that sense time travel is physically possible.

Yet circular time generates paradoxes, familiar from science fiction stories featuring time travel: [ 1 ]

• Consistency: Kurt plans to murder his own grandfather Adolph, by traveling along a CTC to an appropriate moment in the past. He is an able marksman, and waits until he has a clear shot at grandpa. Normally he would not miss. Yet if he succeeds, there is no way that he will then exist to plan and carry out the mission. Kurt pulls the trigger: what can happen?
• Underdetermination: Suppose that Kurt first travels back in order to give his earlier self a copy of How to Build a Time Machine. This is the same book that allows him to build a time machine, which he then carries with him on his journey to the past. Who wrote the book?
• Easy Knowledge: A fan of classical music enhances their computer with a circuit that exploits a CTC. This machine efficiently solves problems at a higher level of computational complexity than conventional computers, leading (among other things) to finding the smallest circuits that can generate Bach’s oeuvre—and to compose new pieces in the same style. Such easy knowledge is at odds with our understanding of our epistemic predicament. (This third paradox has not drawn as much attention.)

The first two paradoxes were once routinely taken to show that solutions with CTCs should be rejected—with charges varying from violating logic, to being “physically unreasonable”, to undermining the notion of free will. Closer analysis of the paradoxes has largely reversed this consensus. Physicists have discovered many solutions with CTCs and have explored their properties in pursuing foundational questions, such as whether physics is compatible with the idea of objective temporal passage (starting with Gödel 1949). Philosophers have also used time travel scenarios to probe questions about, among other things, causation, modality, free will, and identity (see, e.g., Earman 1972 and Lewis’s seminal 1976 paper).

We begin below with Consistency , turning to the other paradoxes in later sections. A standard, stone-walling response is to insist that the past cannot be changed, as a matter of logic, even by a time traveler (e.g., Gödel 1949, Clarke 1977, Horwich 1987). Adolph cannot both die and survive, as a matter of logic, so any scheme to alter the past must fail. In many of the best time travel fictions, the actions of a time traveler are constrained in novel and unexpected ways. Attempts to change the past fail, and they fail, often tragically, in just such a way that they set the stage for the time traveler’s self-defeating journey. The first question is whether there is an analog of the consistent story when it comes to physics in the presence of CTCs. As we will see, there is a remarkable general argument establishing the existence of consistent solutions. Yet a second question persists: why can’t time-traveling Kurt kill his own grandfather? Doesn’t the necessity of failures to change the past put unusual and unexpected constraints on time travelers, or objects that move along CTCs? The same argument shows that there are in fact no constraints imposed by the existence of CTCs, in some cases. After discussing this line of argument, we will turn to the palatability and further implications of such constraints if they are required, and then turn to the implications of quantum mechanics.

Wheeler and Feynman (1949) were the first to claim that the fact that nature is continuous could be used to argue that causal influences from later events to earlier events, as are made possible by time travel, will not lead to paradox without the need for any constraints. Maudlin (1990) showed how to make their argument precise and more general, and argued that nonetheless it was not completely general.

Imagine the following set-up. We start off having a camera with a black and white film ready to take a picture of whatever comes out of the time machine. An object, in fact a developed film, comes out of the time machine. We photograph it, and develop the film. The developed film is subsequently put in the time machine, and set to come out of the time machine at the time the picture is taken. This surely will create a paradox: the developed film will have the opposite distribution of black, white, and shades of gray, from the object that comes out of the time machine. For developed black and white films (i.e., negatives) have the opposite shades of gray from the objects they are pictures of. But since the object that comes out of the time machine is the developed film itself it we surely have a paradox.

However, it does not take much thought to realize that there is no paradox here. What will happen is that a uniformly gray picture will emerge, which produces a developed film that has exactly the same uniform shade of gray. No matter what the sensitivity of the film is, as long as the dependence of the brightness of the developed film depends in a continuous manner on the brightness of the object being photographed, there will be a shade of gray that, when photographed, will produce exactly the same shade of gray on the developed film. This is the essence of Wheeler and Feynman’s idea. Let us first be a bit more precise and then a bit more general.

For simplicity let us suppose that the film is always a uniform shade of gray (i.e., at any time the shade of gray does not vary by location on the film). The possible shades of gray of the film can then be represented by the (real) numbers from 0, representing pure black, to 1, representing pure white.

Let us now distinguish various stages in the chronological order of the life of the film. In stage \(S_1\) the film is young; it has just been placed in the camera and is ready to be exposed. It is then exposed to the object that comes out of the time machine. (That object in fact is a later stage of the film itself). By the time we come to stage \(S_2\) of the life of the film, it has been developed and is about to enter the time machine. Stage \(S_3\) occurs just after it exits the time machine and just before it is photographed. Stage \(S_4\) occurs after it has been photographed and before it starts fading away. Let us assume that the film starts out in stage \(S_1\) in some uniform shade of gray, and that the only significant change in the shade of gray of the film occurs between stages \(S_1\) and \(S_2\). During that period it acquires a shade of gray that depends on the shade of gray of the object that was photographed. In other words, the shade of gray that the film acquires at stage \(S_2\) depends on the shade of gray it has at stage \(S_3\). The influence of the shade of gray of the film at stage \(S_3\), on the shade of gray of the film at stage \(S_2\), can be represented as a mapping, or function, from the real numbers between 0 and 1 (inclusive), to the real numbers between 0 and 1 (inclusive). Let us suppose that the process of photography is such that if one imagines varying the shade of gray of an object in a smooth, continuous manner then the shade of gray of the developed picture of that object will also vary in a smooth, continuous manner. This implies that the function in question will be a continuous function. Now any continuous function from the real numbers between 0 and 1 (inclusive) to the real numbers between 0 and 1 (inclusive) must map at least one number to itself. One can quickly convince oneself of this by graphing such functions. For one will quickly see that any continuous function \(f\) from \([0,1]\) to \([0,1]\) must intersect the line \(x=y\) somewhere, and thus there must be at least one point \(x\) such that \(f(x)=x\). Such points are called fixed points of the function. Now let us think about what such a fixed point represents. It represents a shade of gray such that, when photographed, it will produce a developed film with exactly that same shade of gray. The existence of such a fixed point implies a solution to the apparent paradox.

Let us now be more general and allow color photography. One can represent each possible color of an object (of uniform color) by the proportions of blue, green and red that make up that color. (This is why television screens can produce all possible colors.) Thus one can represent all possible colors of an object by three points on three orthogonal lines \(x, y\) and \(z\), that is to say, by a point in a three-dimensional cube. This cube is also known as the “Cartesian product” of the three line segments. Now, one can also show that any continuous map from such a cube to itself must have at least one fixed point. So color photography can not be used to create time travel paradoxes either!

Even more generally, consider some system \(P\) which, as in the above example, has the following life. It starts in some state \(S_1\), it interacts with an object that comes out of a time machine (which happens to be its older self), it travels back in time, it interacts with some object (which happens to be its younger self), and finally it grows old and dies. Let us assume that the set of possible states of \(P\) can be represented by a Cartesian product of \(n\) closed intervals of the reals, i.e., let us assume that the topology of the state-space of \(P\) is isomorphic to a finite Cartesian product of closed intervals of the reals. Let us further assume that the development of \(P\) in time, and the dependence of that development on the state of objects that it interacts with, is continuous. Then, by a well-known fixed point theorem in topology (see, e.g., Hocking & Young 1961: 273), no matter what the nature of the interaction is, and no matter what the initial state of the object is, there will be at least one state \(S_3\) of the older system (as it emerges from the time travel machine) that will influence the initial state \(S_1\) of the younger system (when it encounters the older system) so that, as the younger system becomes older, it develops exactly into state \(S_3\). Thus without imposing any constraints on the initial state \(S_1\) of the system \(P\), we have shown that there will always be perfectly ordinary, non-paradoxical, solutions, in which everything that happens, happens according to the usual laws of development. Of course, there is looped causation, hence presumably also looped explanation, but what do you expect if there is looped time?

Unfortunately, for the fan of time travel, a little reflection suggests that there are systems for which the needed fixed point theorem does not hold. Imagine, for instance, that we have a dial that can only rotate in a plane. We are going to put the dial in the time machine. Indeed we have decided that if we see the later stage of the dial come out of the time machine set at angle \(x\), then we will set the dial to \(x+90\), and throw it into the time machine. Now it seems we have a paradox, since the mapping that consists of a rotation of all points in a circular state-space by 90 degrees does not have a fixed point. And why wouldn’t some state-spaces have the topology of a circle?

However, we have so far not used another continuity assumption which is also a reasonable assumption. So far we have only made the following demand: the state the dial is in at stage \(S_2\) must be a continuous function of the state of the dial at stage \(S_3\). But, the state of the dial at stage \(S_2\) is arrived at by taking the state of the dial at stage \(S_1\), and rotating it over some angle. It is not merely the case that the effect of the interaction, namely the state of the dial at stage \(S_2\), should be a continuous function of the cause, namely the state of the dial at stage \(S_3\). It is additionally the case that path taken to get there, the way the dial is rotated between stages \(S_1\) and \(S_2\) must be a continuous function of the state at stage \(S_3\). And, rather surprisingly, it turns out that this can not be done. Let us illustrate what the problem is before going to a more general demonstration that there must be a fixed point solution in the dial case.

Forget time travel for the moment. Suppose that you and I each have a watch with a single dial neither of which is running. My watch is set at 12. You are going to announce what your watch is set at. My task is going to be to adjust my watch to yours no matter what announcement you make. And my actions should have a continuous (single valued) dependence on the time that you announce. Surprisingly, this is not possible! For instance, suppose that if you announce “12”, then I achieve that setting on my watch by doing nothing. Now imagine slowly and continuously increasing the announced times, starting at 12. By continuity, I must achieve each of those settings by rotating my dial to the right. If at some point I switch and achieve the announced goal by a rotation of my dial to the left, I will have introduced a discontinuity in my actions, a discontinuity in the actions that I take as a function of the announced angle. So I will be forced, by continuity, to achieve every announcement by rotating the dial to the right. But, this rotation to the right will have to be abruptly discontinued as the announcements grow larger and I eventually approach 12 again, since I achieved 12 by not rotating the dial at all. So, there will be a discontinuity at 12 at the latest. In general, continuity of my actions as a function of announced times can not be maintained throughout if I am to be able to replicate all possible settings. Another way to see the problem is that one can similarly reason that, as one starts with 12, and imagines continuously making the announced times earlier, one will be forced, by continuity, to achieve the announced times by rotating the dial to the left. But the conclusions drawn from the assumption of continuous increases and the assumption of continuous decreases are inconsistent. So we have an inconsistency following from the assumption of continuity and the assumption that I always manage to set my watch to your watch. So, a dial developing according to a continuous dynamics from a given initial state, can not be set up so as to react to a second dial, with which it interacts, in such a way that it is guaranteed to always end up set at the same angle as the second dial. Similarly, it can not be set up so that it is guaranteed to always end up set at 90 degrees to the setting of the second dial. All of this has nothing to do with time travel. However, the impossibility of such set ups is what prevents us from enacting the rotation by 90 degrees that would create paradox in the time travel setting.

Let us now give the positive result that with such dials there will always be fixed point solutions, as long as the dynamics is continuous. Let us call the state of the dial before it interacts with its older self the initial state of the dial. And let us call the state of the dial after it emerges from the time machine the final state of the dial. There is also an intermediate state of the dial, after it interacts with its older self and before it is put into the time machine. We can represent the initial or intermediate states of the dial, before it goes into the time machine, as an angle \(x\) in the horizontal plane and the final state of the dial, after it comes out of the time machine, as an angle \(y\) in the vertical plane. All possible \(\langle x,y\rangle\) pairs can thus be visualized as a torus with each \(x\) value picking out a vertical circular cross-section and each \(y\) picking out a point on that cross-section. See figure 1 .

Figure 1 [An extended description of figure 1 is in the supplement.]

Suppose that the dial starts at angle \(i\) which picks out vertical circle \(I\) on the torus. The initial angle \(i\) that the dial is at before it encounters its older self, and the set of all possible final angles that the dial can have when it emerges from the time machine is represented by the circle \(I\) on the torus (see figure 1 ). Given any possible angle of the emerging dial, the dial initially at angle \(i\) will develop to some other angle. One can picture this development by rotating each point on \(I\) in the horizontal direction by the relevant amount. Since the rotation has to depend continuously on the angle of the emerging dial, circle \(I\) during this development will deform into some loop \(L\) on the torus. Loop \(L\) thus represents all possible intermediate angles \(x\) that the dial is at when it is thrown into the time machine, given that it started at angle \(i\) and then encountered a dial (its older self) which was at angle \(y\) when it emerged from the time machine. We therefore have consistency if \(x=y\) for some \(x\) and \(y\) on loop \(L\). Now, let loop \(C\) be the loop which consists of all the points on the torus for which \(x=y\). Ring \(I\) intersects \(C\) at point \(\langle i,i\rangle\). Obviously any continuous deformation of \(I\) must still intersect \(C\) somewhere. So \(L\) must intersect \(C\) somewhere, say at \(\langle j,j\rangle\). But that means that no matter how the development of the dial starting at \(I\) depends on the angle of the emerging dial, there will be some angle for the emerging dial such that the dial will develop exactly into that angle (by the time it enters the time machine) under the influence of that emerging dial. This is so no matter what angle one starts with, and no matter how the development depends on the angle of the emerging dial. Thus even for a circular state-space there are no constraints needed other than continuity.

Unfortunately there are state-spaces that escape even this argument. Consider for instance a pointer that can be set to all values between 0 and 1, where 0 and 1 are not possible values. That is, suppose that we have a state-space that is isomorphic to an open set of real numbers. Now suppose that we have a machine that sets the pointer to half the value that the pointer is set at when it emerges from the time machine.

Figure 2 [An extended description of figure 2 is in the supplement.]

Suppose the pointer starts at value \(I\). As before we can represent the combination of this initial position and all possible final positions by the line \(I\). Under the influence of the pointer coming out of the time machine the pointer value will develop to a value that equals half the value of the final value that it encountered. We can represent this development as the continuous deformation of line \(I\) into line \(L\), which is indicated by the arrows in figure 2 . This development is fully continuous. Points \(\langle x,y\rangle\) on line \(I\) represent the initial position \(x=I\) of the (young) pointer, and the position \(y\) of the older pointer as it emerges from the time machine. Points \(\langle x,y\rangle\) on line \(L\) represent the position \(x\) that the younger pointer should develop into, given that it encountered the older pointer emerging from the time machine set at position \(y\). Since the pointer is designed to develop to half the value of the pointer that it encounters, the line \(L\) corresponds to \(x=1/2 y\). We have consistency if there is some point such that it develops into that point, if it encounters that point. Thus, we have consistency if there is some point \(\langle x,y\rangle\) on line \(L\) such that \(x=y\). However, there is no such point: lines \(L\) and \(C\) do not intersect. Thus there is no consistent solution, despite the fact that the dynamics is fully continuous.

Of course if 0 were a possible value, \(L\) and \(C\) would intersect at 0. This is surprising and strange: adding one point to the set of possible values of a quantity here makes the difference between paradox and peace. One might be tempted to just add the extra point to the state-space in order to avoid problems. After all, one might say, surely no measurements could ever tell us whether the set of possible values includes that exact point or not. Unfortunately there can be good theoretical reasons for supposing that some quantity has a state-space that is open: the set of all possible speeds of massive objects in special relativity surely is an open set, since it includes all speeds up to, but not including, the speed of light. Quantities that have possible values that are not bounded also lead to counter examples to the presented fixed point argument. And it is not obvious to us why one should exclude such possibilities. So the argument that no constraints are needed is not fully general.

An interesting question of course is: exactly for which state-spaces must there be such fixed points? The arguments above depend on a well-known fixed point theorem (due to Schauder) that guarantees the existence of a fixed point for compact, convex state spaces. We do not know what subsequent extensions of this result imply regarding fixed points for a wider variety of systems, or whether there are other general results along these lines. (See Kutach 2003 for more on this issue.)

A further interesting question is whether this line of argument is sufficient to resolve Consistency (see also Dowe 2007). When they apply, these results establish the existence of a solution, such as the shade of uniform gray in the first example. But physicists routinely demand more than merely the existence of a solution, namely that solutions to the equations are stable—such that “small” changes of the initial state lead to “small” changes of the resulting trajectory. (Clarifying the two senses of “small” in this statement requires further work, specifying the relevant topology.) Stability in this sense underwrites the possibility of applying equations to real systems given our inability to fix initial states with indefinite precision. (See Fletcher 2020 for further discussion.) The fixed point theorems guarantee that for an initial state \(S_1\) there is a solution, but this solution may not be “close” to the solution for a nearby initial state, \(S'\). We are not aware of any proofs that the solutions guaranteed to exist by the fixed point theorems are also stable in this sense.

Time travel has recently been discussed quite extensively in the context of general relativity. General relativity places few constraints on the global structure of space and time. This flexibility leads to a possibility first described in print by Hermann Weyl:

Every world-point is the origin of the double-cone of the active future and the passive past [i.e., the two lobes of the light cone]. Whereas in the special theory of relativity these two portions are separated by an intervening region, it is certainly possible in the present case [i.e., general relativity] for the cone of the active future to overlap with that of the passive past; so that, in principle, it is possible to experience events now that will in part be an effect of my future resolves and actions. Moreover, it is not impossible for a world-line (in particular, that of my body), although it has a timelike direction at every point, to return to the neighborhood of a point which it has already once passed through. (Weyl 1918/1920 [1952: 274])

A time-like curve is simply a space-time trajectory such that the speed of light is never equaled or exceeded along this trajectory. Time-like curves represent possible trajectories of ordinary objects. In general relativity a curve that is everywhere timelike locally can nonetheless loop back on itself, forming a CTC. Weyl makes the point vividly in terms of the light cones: along such a curve, the future lobe of the light cone (the “active future”) intersects the past lobe of the light cone (the “passive past”). Traveling along such a curve one would never exceed the speed of light, and yet after a certain amount of (proper) time one would return to a point in space-time that one previously visited. Or, by staying close to such a CTC, one could come arbitrarily close to a point in space-time that one previously visited. General relativity, in a straightforward sense, allows time travel: there appear to be many space-times compatible with the fundamental equations of general relativity in which there are CTC’s. Space-time, for instance, could have a Minkowski metric everywhere, and yet have CTC’s everywhere by having the temporal dimension (topologically) rolled up as a circle. Or, one can have wormhole connections between different parts of space-time which allow one to enter “mouth \(A\)” of such a wormhole connection, travel through the wormhole, exit the wormhole at “mouth \(B\)” and re-enter “mouth \(A\)” again. CTCs can even arise when the spacetime is topologically \(\mathbb{R}^4\), due to the “tilting” of light cones produced by rotating matter (as in Gödel 1949’s spacetime).

General relativity thus appears to provide ample opportunity for time travel. Note that just because there are CTC’s in a space-time, this does not mean that one can get from any point in the space-time to any other point by following some future directed timelike curve—there may be insurmountable practical obstacles. In Gödel’s spacetime, it is the case that there are CTCs passing through every point in the spacetime. Yet these CTCs are not geodesics, so traversing them requires acceleration. Calculations of the minimal fuel required to travel along the appropriate curve should discourage any would-be time travelers (Malament 1984, 1985; Manchak 2011). But more generally CTCs may be confined to smaller regions; some parts of space-time can have CTC’s while other parts do not. Let us call the part of a space-time that has CTC’s the “time travel region” of that space-time, while calling the rest of that space-time the “normal region”. More precisely, the “time travel region” consists of all the space-time points \(p\) such that there exists a (non-zero length) timelike curve that starts at \(p\) and returns to \(p\). Now let us turn to examining space-times with CTC’s a bit more closely for potential problems.

In order to get a feeling for the sorts of implications that closed timelike curves can have, it may be useful to consider two simple models. In space-times with closed timelike curves the traditional initial value problem cannot be framed in the usual way. For it presupposes the existence of Cauchy surfaces, and if there are CTCs then no Cauchy surface exists. (A Cauchy surface is a spacelike surface such that every inextendable timelike curve crosses it exactly once. One normally specifies initial conditions by giving the conditions on such a surface.) Nonetheless, if the topological complexities of the manifold are appropriately localized, we can come quite close. Let us call an edgeless spacelike surface \(S\) a quasi-Cauchy surface if it divides the rest of the manifold into two parts such that

• every point in the manifold can be connected by a timelike curve to \(S\), and
• any timelike curve which connects a point in one region to a point in the other region intersects \(S\) exactly once.

It is obvious that a quasi-Cauchy surface must entirely inhabit the normal region of the space-time; if any point \(p\) of \(S\) is in the time travel region, then any timelike curve which intersects \(p\) can be extended to a timelike curve which intersects \(S\) near \(p\) again. In extreme cases of time travel, a model may have no normal region at all (e.g., Minkowski space-time rolled up like a cylinder in a time-like direction), in which case our usual notions of temporal precedence will not apply. But temporal anomalies like wormholes (and time machines) can be sufficiently localized to permit the existence of quasi-Cauchy surfaces.

Given a timelike orientation, a quasi-Cauchy surface unproblematically divides the manifold into its past (i.e., all points that can be reached by past-directed timelike curves from \(S)\) and its future (ditto mutatis mutandis ). If the whole past of \(S\) is in the normal region of the manifold, then \(S\) is a partial Cauchy surface : every inextendable timelike curve which exists to the past of \(S\) intersects \(S\) exactly once, but (if there is time travel in the future) not every inextendable timelike curve which exists to the future of \(S\) intersects \(S\). Now we can ask a particularly clear question: consider a manifold which contains a time travel region, but also has a partial Cauchy surface \(S\), such that all of the temporal funny business is to the future of \(S\). If all you could see were \(S\) and its past, you would not know that the space-time had any time travel at all. The question is: are there any constraints on the sort of data which can be put on \(S\) and continued to a global solution of the dynamics which are different from the constraints (if any) on the data which can be put on a Cauchy surface in a simply connected manifold and continued to a global solution? If there is time travel to our future, might we we able to tell this now, because of some implied oddity in the arrangement of present things?

It is not at all surprising that there might be constraints on the data which can be put on a locally space-like surface which passes through the time travel region: after all, we never think we can freely specify what happens on a space-like surface and on another such surface to its future, but in this case the surface at issue lies to its own future. But if there were particular constraints for data on a partial Cauchy surface then we would apparently need to have to rule out some sorts of otherwise acceptable states on \(S\) if there is to be time travel to the future of \(S\). We then might be able to establish that there will be no time travel in the future by simple inspection of the present state of the universe. As we will see, there is reason to suspect that such constraints on the partial Cauchy surface are non-generic. But we are getting ahead of ourselves: first let’s consider the effect of time travel on a very simple dynamics.

The simplest possible example is the Newtonian theory of perfectly elastic collisions among equally massive particles in one spatial dimension. The space-time is two-dimensional, so we can represent it initially as the Euclidean plane, and the dynamics is completely specified by two conditions. When particles are traveling freely, their world lines are straight lines in the space-time, and when two particles collide, they exchange momenta, so the collision looks like an “\(X\)” in space-time, with each particle changing its momentum at the impact. [ 2 ] The dynamics is purely local, in that one can check that a set of world-lines constitutes a model of the dynamics by checking that the dynamics is obeyed in every arbitrarily small region. It is also trivial to generate solutions from arbitrary initial data if there are no CTCs: given the initial positions and momenta of a set of particles, one simply draws a straight line from each particle in the appropriate direction and continues it indefinitely. Once all the lines are drawn, the worldline of each particle can be traced from collision to collision. The boundary value problem for this dynamics is obviously well-posed: any set of data at an instant yields a unique global solution, constructed by the method sketched above.

What happens if we change the topology of the space-time by hand to produce CTCs? The simplest way to do this is depicted in figure 3 : we cut and paste the space-time so it is no longer simply connected by identifying the line \(L-\) with the line \(L+\). Particles “going in” to \(L+\) from below “emerge” from \(L-\) , and particles “going in” to \(L-\) from below “emerge” from \(L+\).

Figure 3: Inserting CTCs by Cut and Paste. [An extended description of figure 3 is in the supplement.]

How is the boundary-value problem changed by this alteration in the space-time? Before the cut and paste, we can put arbitrary data on the simultaneity slice \(S\) and continue it to a unique solution. After the change in topology, \(S\) is no longer a Cauchy surface, since a CTC will never intersect it, but it is a partial Cauchy surface. So we can ask two questions. First, can arbitrary data on \(S\) always be continued to a global solution? Second, is that solution unique? If the answer to the first question is \(no\), then we have a backward-temporal constraint: the existence of the region with CTCs places constraints on what can happen on \(S\) even though that region lies completely to the future of \(S\). If the answer to the second question is \(no\), then we have an odd sort of indeterminism, analogous to the unwritten book: the complete physical state on \(S\) does not determine the physical state in the future, even though the local dynamics is perfectly deterministic and even though there is no other past edge to the space-time region in \(S\)’s future (i.e., there is nowhere else for boundary values to come from which could influence the state of the region).

In this case the answer to the first question is yes and to the second is no : there are no constraints on the data which can be put on \(S\), but those data are always consistent with an infinitude of different global solutions. The easy way to see that there always is a solution is to construct the minimal solution in the following way. Start drawing straight lines from \(S\) as required by the initial data. If a line hits \(L-\) from the bottom, just continue it coming out of the top of \(L+\) in the appropriate place, and if a line hits \(L+\) from the bottom, continue it emerging from \(L-\) at the appropriate place. Figure 4 represents the minimal solution for a single particle which enters the time-travel region from the left:

Figure 4: The Minimal Solution. [An extended description of figure 4 is in the supplement.]

The particle “travels back in time” three times. It is obvious that this minimal solution is a global solution, since the particle always travels inertially.

But the same initial state on \(S\) is also consistent with other global solutions. The new requirement imposed by the topology is just that the data going into \(L+\) from the bottom match the data coming out of \(L-\) from the top, and the data going into \(L-\) from the bottom match the data coming out of \(L+\) from the top. So we can add any number of vertical lines connecting \(L-\) and \(L+\) to a solution and still have a solution. For example, adding a few such lines to the minimal solution yields:

Figure 5: A Non-Minimal Solution. [An extended description of figure 5 is in the supplement.]

The particle now collides with itself twice: first before it reaches \(L+\) for the first time, and again shortly before it exits the CTC region. From the particle’s point of view, it is traveling to the right at a constant speed until it hits an older version of itself and comes to rest. It remains at rest until it is hit from the right by a younger version of itself, and then continues moving off, and the same process repeats later. It is clear that this is a global model of the dynamics, and that any number of distinct models could be generating by varying the number and placement of vertical lines.

Knowing the data on \(S\), then, gives us only incomplete information about how things will go for the particle. We know that the particle will enter the CTC region, and will reach \(L+\), we know that it will be the only particle in the universe, we know exactly where and with what speed it will exit the CTC region. But we cannot determine how many collisions the particle will undergo (if any), nor how long (in proper time) it will stay in the CTC region. If the particle were a clock, we could not predict what time it would indicate when exiting the region. Furthermore, the dynamics gives us no handle on what to think of the various possibilities: there are no probabilities assigned to the various distinct possible outcomes.

Changing the topology has changed the mathematics of the situation in two ways, which tend to pull in opposite directions. On the one hand, \(S\) is no longer a Cauchy surface, so it is perhaps not surprising that data on \(S\) do not suffice to fix a unique global solution. But on the other hand, there is an added constraint: data “coming out” of \(L-\) must exactly match data “going in” to \(L+\), even though what comes out of \(L-\) helps to determine what goes into \(L+\). This added consistency constraint tends to cut down on solutions, although in this case the additional constraint is more than outweighed by the freedom to consider various sorts of data on \({L+}/{L-}\).

The fact that the extra freedom outweighs the extra constraint also points up one unexpected way that the supposed paradoxes of time travel may be overcome. Let’s try to set up a paradoxical situation using the little closed time loop above. If we send a single particle into the loop from the left and do nothing else, we know exactly where it will exit the right side of the time travel region. Now suppose we station someone at the other side of the region with the following charge: if the particle should come out on the right side, the person is to do something to prevent the particle from going in on the left in the first place. In fact, this is quite easy to do: if we send a particle in from the right, it seems that it can exit on the left and deflect the incoming left-hand particle.

Carrying on our reflection in this way, we further realize that if the particle comes out on the right, we might as well send it back in order to deflect itself from entering in the first place. So all we really need to do is the following: set up a perfectly reflecting particle mirror on the right-hand side of the time travel region, and launch the particle from the left so that— if nothing interferes with it —it will just barely hit \(L+\). Our paradox is now apparently complete. If, on the one hand, nothing interferes with the particle it will enter the time-travel region on the left, exit on the right, be reflected from the mirror, re-enter from the right, and come out on the left to prevent itself from ever entering. So if it enters, it gets deflected and never enters. On the other hand, if it never enters then nothing goes in on the left, so nothing comes out on the right, so nothing is reflected back, and there is nothing to deflect it from entering. So if it doesn’t enter, then there is nothing to deflect it and it enters. If it enters, then it is deflected and doesn’t enter; if it doesn’t enter then there is nothing to deflect it and it enters: paradox complete.

But at least one solution to the supposed paradox is easy to construct: just follow the recipe for constructing the minimal solution, continuing the initial trajectory of the particle (reflecting it the mirror in the obvious way) and then read of the number and trajectories of the particles from the resulting diagram. We get the result of figure 6 :

Figure 6: Resolving the “Paradox”. [An extended description of figure 6 is in the supplement.]

As we can see, the particle approaching from the left never reaches \(L+\): it is deflected first by a particle which emerges from \(L-\). But it is not deflected by itself , as the paradox suggests, it is deflected by another particle. Indeed, there are now four particles in the diagram: the original particle and three particles which are confined to closed time-like curves. It is not the leftmost particle which is reflected by the mirror, nor even the particle which deflects the leftmost particle; it is another particle altogether.

The paradox gets it traction from an incorrect presupposition. If there is only one particle in the world at \(S\) then there is only one particle which could participate in an interaction in the time travel region: the single particle would have to interact with its earlier (or later) self. But there is no telling what might come out of \(L-\): the only requirement is that whatever comes out must match what goes in at \(L+\). So if you go to the trouble of constructing a working time machine, you should be prepared for a different kind of disappointment when you attempt to go back and kill yourself: you may be prevented from entering the machine in the first place by some completely unpredictable entity which emerges from it. And once again a peculiar sort of indeterminism appears: if there are many self-consistent things which could prevent you from entering, there is no telling which is even likely to materialize. This is just like the case of the unwritten book: the book is never written, so nothing determines what fills its pages.

So when the freedom to put data on \(L-\) outweighs the constraint that the same data go into \(L+\), instead of paradox we get an embarrassment of riches: many solution consistent with the data on \(S\), or many possible books. To see a case where the constraint “outweighs” the freedom, we need to construct a very particular, and frankly artificial, dynamics and topology. Consider the space of all linear dynamics for a scalar field on a lattice. (The lattice can be though of as a simple discrete space-time.) We will depict the space-time lattice as a directed graph. There is to be a scalar field defined at every node of the graph, whose value at a given node depends linearly on the values of the field at nodes which have arrows which lead to it. Each edge of the graph can be assigned a weighting factor which determines how much the field at the input node contributes to the field at the output node. If we name the nodes by the letters a , b , c , etc., and the edges by their endpoints in the obvious way, then we can label the weighting factors by the edges they are associated with in an equally obvious way.

Suppose that the graph of the space-time lattice is acyclic , as in figure 7 . (A graph is Acyclic if one can not travel in the direction of the arrows and go in a loop.)

Figure 7: An Acyclic Lattice. [An extended description of figure 7 is in the supplement.]

It is easy to regard a set of nodes as the analog of a Cauchy surface, e.g., the set \(\{a, b, c\}\), and it is obvious if arbitrary data are put on those nodes the data will generate a unique solution in the future. [ 3 ] If the value of the field at node \(a\) is 3 and at node \(b\) is 7, then its value at node \(d\) will be \(3W_{ad}\) and its value at node \(e\) will be \(3W_{ae} + 7W_{be}\). By varying the weighting factors we can adjust the dynamics, but in an acyclic graph the future evolution of the field will always be unique.

Let us now again artificially alter the topology of the lattice to admit CTCs, so that the graph now is cyclic. One of the simplest such graphs is depicted in figure 8 : there are now paths which lead from \(z\) back to itself, e.g., \(z\) to \(y\) to \(z\).

Figure 8: Time Travel on a Lattice. [An extended description of figure 8 is in the supplement.]

Can we now put arbitrary data on \(v\) and \(w\), and continue that data to a global solution? Will the solution be unique?

In the generic case, there will be a solution and the solution will be unique. The equations for the value of the field at \(x, y\), and \(z\) are:

Solving these equations for \(z\) yields

which gives a unique value for \(z\) in the generic case. But looking at the space of all possible dynamics for this lattice (i.e., the space of all possible weighting factors), we find a singularity in the case where \(1-W_{zx}W_{xz} - W_{zy}W_{yz} = 0\). If we choose weighting factors in just this way, then arbitrary data at \(v\) and \(w\) cannot be continued to a global solution. Indeed, if the scalar field is everywhere non-negative, then this particular choice of dynamics puts ironclad constraints on the value of the field at \(v\) and \(w\): the field there must be zero (assuming \(W_{vx}\) and \(W_{wy}\) to be non-zero), and similarly all nodes in their past must have field value zero. If the field can take negative values, then the values at \(v\) and \(w\) must be so chosen that \(vW_{vx}W_{xz} = -wW_{wy}W_{yz}\). In either case, the field values at \(v\) and \(w\) are severely constrained by the existence of the CTC region even though these nodes lie completely to the past of that region. It is this sort of constraint which we find to be unlike anything which appears in standard physics.

Our toy models suggest three things. The first is that it may be impossible to prove in complete generality that arbitrary data on a partial Cauchy surface can always be continued to a global solution: our artificial case provides an example where it cannot. The second is that such odd constraints are not likely to be generic: we had to delicately fine-tune the dynamics to get a problem. The third is that the opposite problem, namely data on a partial Cauchy surface being consistent with many different global solutions, is likely to be generic: we did not have to do any fine-tuning to get this result.

This third point leads to a peculiar sort of indeterminism, illustrated by the case of the unwritten book: the entire state on \(S\) does not determine what will happen in the future even though the local dynamics is deterministic and there are no other “edges” to space-time from which data could influence the result. What happens in the time travel region is constrained but not determined by what happens on \(S\), and the dynamics does not even supply any probabilities for the various possibilities. The example of the photographic negative discussed in section 2, then, seems likely to be unusual, for in that case there is a unique fixed point for the dynamics, and the set-up plus the dynamical laws determine the outcome. In the generic case one would rather expect multiple fixed points, with no room for anything to influence, even probabilistically, which would be realized. (See the supplement on

Remarks and Limitations on the Toy Models .

It is ironic that time travel should lead generically not to contradictions or to constraints (in the normal region) but to underdetermination of what happens in the time travel region by what happens everywhere else (an underdetermination tied neither to a probabilistic dynamics nor to a free edge to space-time). The traditional objection to time travel is that it leads to contradictions: there is no consistent way to complete an arbitrarily constructed story about how the time traveler intends to act. Instead, though, it appears that the more significant problem is underdetermination: the story can be consistently completed in many different ways.

Echeverria, Klinkhammer, and Thorne (1991) considered the case of 3-dimensional single hard spherical ball that can go through a single time travel wormhole so as to collide with its younger self.

Figure 9 [An extended description of figure 9 is in the supplement.]

The threat of paradox in this case arises in the following form. Consider the initial trajectory of a ball as it approaches the time travel region. For some initial trajectories, the ball does not undergo a collision before reaching mouth 1, but upon exiting mouth 2 it will collide with its earlier self. This leads to a contradiction if the collision is strong enough to knock the ball off its trajectory and deflect it from entering mouth 1. Of course, the Wheeler-Feynman strategy is to look for a “glancing blow” solution: a collision which will produce exactly the (small) deviation in trajectory of the earlier ball that produces exactly that collision. Are there always such solutions? [ 4 ]

Echeverria, Klinkhammer & Thorne found a large class of initial trajectories that have consistent “glancing blow” continuations, and found none that do not (but their search was not completely general). They did not produce a rigorous proof that every initial trajectory has a consistent continuation, but suggested that it is very plausible that every initial trajectory has a consistent continuation. That is to say, they have made it very plausible that, in the billiard ball wormhole case, the time travel structure of such a wormhole space-time does not result in constraints on states on spacelike surfaces in the non-time travel region.

In fact, as one might expect from our discussion in the previous section, they found the opposite problem from that of inconsistency: they found underdetermination. For a large class of initial trajectories there are multiple different consistent “glancing blow” continuations of that trajectory (many of which involve multiple wormhole traversals). For example, if one initially has a ball that is traveling on a trajectory aimed straight between the two mouths, then one obvious solution is that the ball passes between the two mouths and never time travels. But another solution is that the younger ball gets knocked into mouth 1 exactly so as to come out of mouth 2 and produce that collision. Echeverria et al. do not note the possibility (which we pointed out in the previous section) of the existence of additional balls in the time travel region. We conjecture (but have no proof) that for every initial trajectory of \(A\) there are some, and generically many, multiple-ball continuations.

Friedman, Morris, et al. (1990) examined the case of source-free non-self-interacting scalar fields traveling through such a time travel wormhole and found that no constraints on initial conditions in the non-time travel region are imposed by the existence of such time travel wormholes. In general there appear to be no known counter examples to the claim that in “somewhat realistic” time-travel space-times with a partial Cauchy surface there are no constraints imposed on the state on such a partial Cauchy surface by the existence of CTC’s. (See, e.g., Friedman & Morris 1991; Thorne 1994; Earman 1995; Earman, Smeenk, & Wüthrich 2009; and Dowe 2007.)

How about the issue of constraints in the time travel region \(T\)? Prima facie , constraints in such a region would not appear to be surprising. But one might still expect that there should be no constraints on states on a spacelike surface, provided one keeps the surface “small enough”. In the physics literature the following question has been asked: for any point \(p\) in \(T\), and any space-like surface \(S\) that includes \(p\) is there a neighborhood \(E\) of \(p\) in \(S\) such that any solution on \(E\) can be extended to a solution on the whole space-time? With respect to this question, there are some simple models in which one has this kind of extendability of local solutions to global ones, and some simple models in which one does not have such extendability, with no clear general pattern. The technical mathematical problems are amplified by the more conceptual problem of what it might mean to say that one could create a situation which forces the creation of closed timelike curves. (See, e.g., Yurtsever 1990; Friedman, Morris, et al. 1990; Novikov 1992; Earman 1995; and Earman, Smeenk, & Wüthrich 2009). What are we to think of all of this?

The toy models above all treat billiard balls, fields, and other objects propagating through a background spacetime with CTCs. Even if we can show that a consistent solution exists, there is a further question: what kind of matter and dynamics could generate CTCs to begin with? There are various solutions of Einstein’s equations with CTCs, but how do these exotic spacetimes relate to the models actually used in describing the world? In other words, what positive reasons might we have to take CTCs seriously as a feature of the actual universe, rather than an exotic possibility of primarily mathematical interest?

We should distinguish two different kinds of “possibility” that we might have in mind in posing such questions (following Stein 1970). First, we can consider a solution as a candidate cosmological model, describing the (large-scale gravitational degrees of freedom of the) entire universe. The case for ruling out spacetimes with CTCs as potential cosmological models strikes us as, surprisingly, fairly weak. Physicists used to simply rule out solutions with CTCs as unreasonable by fiat, due to the threat of paradoxes, which we have dismantled above. But it is also challenging to make an observational case. Observations tell us very little about global features, such as the existence of CTCs, because signals can only reach an observer from a limited region of spacetime, called the past light cone. Our past light cone—and indeed the collection of all the past light cones for possible observers in a given spacetime—can be embedded in spacetimes with quite different global features (Malament 1977, Manchak 2009). This undercuts the possibility of using observations to constrain global topology, including (among other things) ruling out the existence of CTCs.

Yet the case in favor of taking cosmological models with CTCs seriously is also not particularly strong. Some solutions used to describe black holes, which are clearly relevant in a variety of astrophysical contexts, include CTCs. But the question of whether the CTCs themselves play an essential representational role is subtle: the CTCs arise in the maximal extensions of these solutions, and can plausibly be regarded as extraneous to successful applications. Furthermore, many of the known solutions with CTCs have symmetries, raising the possibility that CTCs are not a stable or robust feature. Slight departures from symmetry may lead to a solution without CTCs, suggesting that the CTCs may be an artifact of an idealized model.

The second sense of possibility regards whether “reasonable” initial conditions can be shown to lead to, or not to lead to, the formation of CTCs. As with the toy models above, suppose that we have a partial Cauchy surface \(S\), such that all the temporal funny business lies to the future. Rather than simply assuming that there is a region with CTCs to the future, we can ask instead whether it is possible to create CTCs by manipulating matter in the initial, well-behaved region—that is, whether it is possible to build a time machine. Several physicists have pursued “chronology protection theorems” aiming to show that the dynamics of general relativity (or some other aspects of physics) rules this out, and to clarify why this is the case. The proof of such a theorem would justify neglecting solutions with CTCs as a source of insight into the nature of time in the actual world. But as of yet there are several partial results that do not fully settle the question. One further intriguing possibility is that even if general relativity by itself does protect chronology, it may not be possible to formulate a sensible theory describing matter and fields in solutions with CTCs. (See SEP entry on Time Machines; Smeenk and Wüthrich 2011 for more.)

There is a different question regarding the limitations of these toy models. The toy models and related examples show that there are consistent solutions for simple systems in the presence of CTCs. As usual we have made the analysis tractable by building toy models, selecting only a few dynamical degrees of freedom and tracking their evolution. But there is a large gap between the systems we have described and the time travel stories they evoke, with Kurt traveling along a CTC with murderous intentions. In particular, many features of the manifest image of time are tied to the thermodynamical properties of macroscopic systems. Rovelli (unpublished) considers a extremely simple system to illustrate the problem: can a clock move along a CTC? A clock consists of something in periodic motion, such as a pendulum bob, and something that counts the oscillations, such as an escapement mechanism. The escapement mechanism cannot work without friction; this requires dissipation and increasing entropy. For a clock that counts oscillations as it moves along a time-like trajectory, the entropy must be a monotonically increasing function. But that is obviously incompatible with the clock returning to precisely the same state at some future time as it completes a loop. The point generalizes, obviously, to imply that anything like a human, with memory and agency, cannot move along a CTC.

Since it is not obvious that one can rid oneself of all constraints in realistic models, let us examine the argument that time travel is implausible, and we should think it unlikely to exist in our world, in so far as it implies such constraints. The argument goes something like the following. In order to satisfy such constraints one needs some pre-established divine harmony between the global (time travel) structure of space-time and the distribution of particles and fields on space-like surfaces in it. But it is not plausible that the actual world, or any world even remotely like ours, is constructed with divine harmony as part of the plan. In fact, one might argue, we have empirical evidence that conditions in any spatial region can vary quite arbitrarily. So we have evidence that such constraints, whatever they are, do not in fact exist in our world. So we have evidence that there are no closed time-like lines in our world or one remotely like it. We will now examine this argument in more detail by presenting four possible responses, with counterresponses, to this argument.

Response 1. There is nothing implausible or new about such constraints. For instance, if the universe is spatially closed, there has to be enough matter to produce the needed curvature, and this puts constraints on the matter distribution on a space-like hypersurface. Thus global space-time structure can quite unproblematically constrain matter distributions on space-like hypersurfaces in it. Moreover we have no realistic idea what these constraints look like, so we hardly can be said to have evidence that they do not obtain.

Counterresponse 1. Of course there are constraining relations between the global structure of space-time and the matter in it. The Einstein equations relate curvature of the manifold to the matter distribution in it. But what is so strange and implausible about the constraints imposed by the existence of closed time-like curves is that these constraints in essence have nothing to do with the Einstein equations. When investigating such constraints one typically treats the particles and/or field in question as test particles and/or fields in a given space-time, i.e., they are assumed not to affect the metric of space-time in any way. In typical space-times without closed time-like curves this means that one has, in essence, complete freedom of matter distribution on a space-like hypersurface. (See response 2 for some more discussion of this issue). The constraints imposed by the possibility of time travel have a quite different origin and are implausible. In the ordinary case there is a causal interaction between matter and space-time that results in relations between global structure of space-time and the matter distribution in it. In the time travel case there is no such causal story to be told: there simply has to be some pre-established harmony between the global space-time structure and the matter distribution on some space-like surfaces. This is implausible.

Response 2. Constraints upon matter distributions are nothing new. For instance, Maxwell’s equations constrain electric fields \(\boldsymbol{E}\) on an initial surface to be related to the (simultaneous) charge density distribution \(\varrho\) by the equation \(\varrho = \text{div}(\boldsymbol{E})\). (If we assume that the \(E\) field is generated solely by the charge distribution, this conditions amounts to requiring that the \(E\) field at any point in space simply be the one generated by the charge distribution according to Coulomb’s inverse square law of electrostatics.) This is not implausible divine harmony. Such constraints can hold as a matter of physical law. Moreover, if we had inferred from the apparent free variation of conditions on spatial regions that there could be no such constraints we would have mistakenly inferred that \(\varrho = \text{div}(\boldsymbol{E})\) could not be a law of nature.

Counterresponse 2. The constraints imposed by the existence of closed time-like lines are of quite a different character from the constraint imposed by \(\varrho = \text{div}(\boldsymbol{E})\). The constraints imposed by \(\varrho = \text{div}(\boldsymbol{E})\) on the state on a space-like hypersurface are:

• local constraints (i.e., to check whether the constraint holds in a region you just need to see whether it holds at each point in the region),
• quite independent of the global space-time structure,
• quite independent of how the space-like surface in question is embedded in a given space-time, and
• very simply and generally stateable.

On the other hand, the consistency constraints imposed by the existence of closed time-like curves (i) are not local, (ii) are dependent on the global structure of space-time, (iii) depend on the location of the space-like surface in question in a given space-time, and (iv) appear not to be simply stateable other than as the demand that the state on that space-like surface embedded in such and such a way in a given space-time, do not lead to inconsistency. On some views of laws (e.g., David Lewis’ view) this plausibly implies that such constraints, even if they hold, could not possibly be laws. But even if one does not accept such a view of laws, one could claim that the bizarre features of such constraints imply that it is implausible that such constraints hold in our world or in any world remotely like ours.

Response 3. It would be strange if there are constraints in the non-time travel region. It is not strange if there are constraints in the time travel region. They should be explained in terms of the strange, self-interactive, character of time travel regions. In this region there are time-like trajectories from points to themselves. Thus the state at such a point, in such a region, will, in a sense, interact with itself. It is a well-known fact that systems that interact with themselves will develop into an equilibrium state, if there is such an equilibrium state, or else will develop towards some singularity. Normally, of course, self-interaction isn’t true instantaneous self-interaction, but consists of a feed-back mechanism that takes time. But in time travel regions something like true instantaneous self-interaction occurs. This explains why constraints on states occur in such time travel regions: the states “ ab initio ” have to be “equilibrium states”. Indeed in a way this also provides some picture of why indeterminism occurs in time travel regions: at the onset of self-interaction states can fork into different equi-possible equilibrium states.

Counterresponse 3. This is explanation by woolly analogy. It all goes to show that time travel leads to such bizarre consequences that it is unlikely that it occurs in a world remotely like ours.

Response 4. All of the previous discussion completely misses the point. So far we have been taking the space-time structure as given, and asked the question whether a given time travel space-time structure imposes constraints on states on (parts of) space-like surfaces. However, space-time and matter interact. Suppose that one is in a space-time with closed time-like lines, such that certain counterfactual distributions of matter on some neighborhood of a point \(p\) are ruled out if one holds that space-time structure fixed. One might then ask

Why does the actual state near \(p\) in fact satisfy these constraints? By what divine luck or plan is this local state compatible with the global space-time structure? What if conditions near \(p\) had been slightly different?

And one might take it that the lack of normal answers to these questions indicates that it is very implausible that our world, or any remotely like it, is such a time travel universe. However the proper response to these question is the following. There are no constraints in any significant sense. If they hold they hold as a matter of accidental fact, not of law. There is no more explanation of them possible than there is of any contingent fact. Had conditions in a neighborhood of \(p\) been otherwise, the global structure of space-time would have been different. So what? The only question relevant to the issue of constraints is whether an arbitrary state on an arbitrary spatial surface \(S\) can always be embedded into a space-time such that that state on \(S\) consistently extends to a solution on the entire space-time.

But we know the answer to that question. A well-known theorem in general relativity says the following: any initial data set on a three dimensional manifold \(S\) with positive definite metric has a unique embedding into a maximal space-time in which \(S\) is a Cauchy surface (see, e.g., Geroch & Horowitz 1979: 284 for more detail), i.e., there is a unique largest space-time which has \(S\) as a Cauchy surface and contains a consistent evolution of the initial value data on \(S\). Now since \(S\) is a Cauchy surface this space-time does not have closed time like curves. But it may have extensions (in which \(S\) is not a Cauchy surface) which include closed timelike curves, indeed it may be that any maximal extension of it would include closed timelike curves. (This appears to be the case for extensions of states on certain surfaces of Taub-NUT space-times. See Earman, Smeenk, & Wüthrich 2009). But these extensions, of course, will be consistent. So properly speaking, there are no constraints on states on space-like surfaces. Nonetheless the space-time in which these are embedded may or may not include closed time-like curves.

Counterresponse 4. This, in essence, is the stonewalling answer which we indicated in section 1. However, whether or not you call the constraints imposed by a given space-time on distributions of matter on certain space-like surfaces “genuine constraints”, whether or not they can be considered lawlike, and whether or not they need to be explained, the existence of such constraints can still be used to argue that time travel worlds are so bizarre that it is implausible that our world or any world remotely like ours is a time travel world.

Suppose that one is in a time travel world. Suppose that given the global space-time structure of this world, there are constraints imposed upon, say, the state of motion of a ball on some space-like surface when it is treated as a test particle, i.e., when it is assumed that the ball does not affect the metric properties of the space-time it is in. (There is lots of other matter that, via the Einstein equation, corresponds exactly to the curvature that there is everywhere in this time travel worlds.) Now a real ball of course does have some effect on the metric of the space-time it is in. But let us consider a ball that is so small that its effect on the metric is negligible. Presumably it will still be the case that certain states of this ball on that space-like surface are not compatible with the global time travel structure of this universe.

This means that the actual distribution of matter on such a space-like surface can be extended into a space-time with closed time-like lines, but that certain counterfactual distributions of matter on this space-like surface can not be extended into the same space-time. But note that the changes made in the matter distribution (when going from the actual to the counterfactual distribution) do not in any non-negligible way affect the metric properties of the space-time. (Recall that the changes only effect test particles.) Thus the reason why the global time travel properties of the counterfactual space-time have to be significantly different from the actual space-time is not that there are problems with metric singularities or alterations in the metric that force significant global changes when we go to the counterfactual matter distribution. The reason that the counterfactual space-time has to be different is that in the counterfactual world the ball’s initial state of motion starting on the space-like surface, could not “meet up” in a consistent way with its earlier self (could not be consistently extended) if we were to let the global structure of the counterfactual space-time be the same as that of the actual space-time. Now, it is not bizarre or implausible that there is a counterfactual dependence of manifold structure, even of its topology, on matter distributions on spacelike surfaces. For instance, certain matter distributions may lead to singularities, others may not. We may indeed in some sense have causal power over the topology of the space-time we live in. But this power normally comes via the Einstein equations. But it is bizarre to think that there could be a counterfactual dependence of global space-time structure on the arrangement of certain tiny bits of matter on some space-like surface, where changes in that arrangement by assumption do not affect the metric anywhere in space-time in any significant way . It is implausible that we live in such a world, or that a world even remotely like ours is like that.

Let us illustrate this argument in a different way by assuming that wormhole time travel imposes constraints upon the states of people prior to such time travel, where the people have so little mass/energy that they have negligible effect, via the Einstein equation, on the local metric properties of space-time. Do you think it more plausible that we live in a world where wormhole time travel occurs but it only occurs when people’s states are such that these local states happen to combine with time travel in such a way that nobody ever succeeds in killing their younger self, or do you think it more plausible that we are not in a wormhole time travel world? [ 5 ]

An alternative approach to time travel (initiated by Deutsch 1991) abstracts away from the idealized toy models described above. [ 6 ] This computational approach considers instead the evolution of bits (simple physical systems with two discrete states) through a network of interactions, which can be represented by a circuit diagram with gates corresponding to the interactions. Motivated by the possibility of CTCs, Deutsch proposed adding a new kind of channel that connects the output of a given gate back to its input —in essence, a backwards-time step. More concretely, given a gate that takes \(n\) bits as input, we can imagine taking some number \(i \lt n\) of these bits through a channel that loops back and then do double-duty as inputs. Consistency requires that the state of these \(i\) bits is the same for output and input. (We will consider an illustration of this kind of system in the next section.) Working through examples of circuit diagrams with a CTC channel leads to similar treatments of Consistency and Underdetermination as the discussion above (see, e.g., Wallace 2012: § 10.6). But the approach offers two new insights (both originally due to Deutsch): the Easy Knowledge paradox, and a particularly clear extension to time travel in quantum mechanics.

A computer equipped with a CTC channel can exploit the need to find consistent evolution to solve remarkably hard problems. (This is quite different than the first idea that comes to mind to enhance computational power: namely to just devote more time to a computation, and then send the result back on the CTC to an earlier state.) The gate in a circuit incorporating a CTC implements a function from the input bits to the output bits, under the constraint that the output and input match the i bits going through the CTC channel. This requires, in effect, finding the fixed point of the relevant function. Given the generality of the model, there are few limits on the functions that could be implemented on the CTC circuit. Nature has to solve a hard computational problem just to ensure consistent evolution. This can then be extended to other complex computational problems—leading, more precisely, to solutions of NP -complete problems in polynomial time (see Aaronson 2013: Chapter 20 for an overview and further references). The limits imposed by computational complexity are an essential part of our epistemic situation, and computers with CTCs would radically change this.

We now turn to the application of the computational approach to the quantum physics of time travel (see Deutsch 1991; Deutsch & Lockwood 1994). By contrast with the earlier discussions of constraints in classical systems, they claim to show that time travel never imposes any constraints on the pre-time travel state of quantum systems. The essence of this account is as follows. [ 7 ]

A quantum system starts in state \(S_1\), interacts with its older self, after the interaction is in state \(S_2\), time travels while developing into state \(S_3\), then interacts with its younger self, and ends in state \(S_4\) (see figure 10 ).

Figure 10 [An extended description of figure 10 is in the supplement.]

Deutsch assumes that the set of possible states of this system are the mixed states, i.e., are represented by the density matrices over the Hilbert space of that system. Deutsch then shows that for any initial state \(S_1\), any unitary interaction between the older and younger self, and any unitary development during time travel, there is a consistent solution, i.e., there is at least one pair of states \(S_2\) and \(S_3\) such that when \(S_1\) interacts with \(S_3\) it will change to state \(S_2\) and \(S_2\) will then develop into \(S_3\). The states \(S_2, S_3\) and \(S_4\) will typically be not be pure states, i.e., will be non-trivial mixed states, even if \(S_1\) is pure. In order to understand how this leads to interpretational problems let us give an example. Consider a system that has a two dimensional Hilbert space with as a basis the states \(\vc{+}\) and \(\vc{-}\). Let us suppose that when state \(\vc{+}\) of the young system encounters state \(\vc{+}\) of the older system, they interact and the young system develops into state \(\vc{-}\) and the old system remains in state \(\vc{+}\). In obvious notation:

Similarly, suppose that:

Let us furthermore assume that there is no development of the state of the system during time travel, i.e., that \(\vc{+}_2\) develops into \(\vc{+}_3\), and that \(\vc{-}_2\) develops into \(\vc{-}_3\).

Now, if the only possible states of the system were \(\vc{+}\) and \(\vc{-}\) (i.e., if there were no superpositions or mixtures of these states), then there is a constraint on initial states: initial state \(\vc{+}_1\) is impossible. For if \(\vc{+}_1\) interacts with \(\vc{+}_3\) then it will develop into \(\vc{-}_2\), which, during time travel, will develop into \(\vc{-}_3\), which inconsistent with the assumed state \(\vc{+}_3\). Similarly if \(\vc{+}_1\) interacts with \(\vc{-}_3\) it will develop into \(\vc{+}_2\), which will then develop into \(\vc{+}_3\) which is also inconsistent. Thus the system can not start in state \(\vc{+}_1\).

But, says Deutsch, in quantum mechanics such a system can also be in any mixture of the states \(\vc{+}\) and \(\vc{-}\). Suppose that the older system, prior to the interaction, is in a state \(S_3\) which is an equal mixture of 50% \(\vc{+}_3\) and 50% \(\vc{-}_3\). Then the younger system during the interaction will develop into a mixture of 50% \(\vc{+}_2\) and 50% \(\vc{-}_2\), which will then develop into a mixture of 50% \(\vc{+}_3\) and 50% \(\vc{-}_3\), which is consistent! More generally Deutsch uses a fixed point theorem to show that no matter what the unitary development during interaction is, and no matter what the unitary development during time travel is, for any state \(S_1\) there is always a state \(S_3\) (which typically is not a pure state) which causes \(S_1\) to develop into a state \(S_2\) which develops into that state \(S_3\). Thus quantum mechanics comes to the rescue: it shows in all generality that no constraints on initial states are needed!

One might wonder why Deutsch appeals to mixed states: will superpositions of states \(\vc{+}\) and \(\vc{-}\) not suffice? Unfortunately such an idea does not work. Suppose again that the initial state is \(\vc{+}_1\). One might suggest that that if state \(S_3\) is

one will obtain a consistent development. For one might think that when initial state \(\vc{+}_1\) encounters the superposition

it will develop into superposition

and that this in turn will develop into

as desired. However this is not correct. For initial state \(\vc{+}_1\) when it encounters

will develop into the entangled state

In so far as one can speak of the state of the young system after this interaction, it is in the mixture of 50% \(\vc{+}_2\) and 50% \(\vc{-}_2\), not in the superposition

So Deutsch does need his recourse to mixed states.

This clarification of why Deutsch needs his mixtures does however indicate a serious worry about the simplifications that are part of Deutsch’s account. After the interaction the old and young system will (typically) be in an entangled state. Although for purposes of a measurement on one of the two systems one can say that this system is in a mixed state, one can not represent the full state of the two systems by specifying the mixed state of each separate part, as there are correlations between observables of the two systems that are not represented by these two mixed states, but are represented in the joint entangled state. But if there really is an entangled state of the old and young systems directly after the interaction, how is one to represent the subsequent development of this entangled state? Will the state of the younger system remain entangled with the state of the older system as the younger system time travels and the older system moves on into the future? On what space-like surfaces are we to imagine this total entangled state to be? At this point it becomes clear that there is no obvious and simple way to extend elementary non-relativistic quantum mechanics to space-times with closed time-like curves: we apparently need to characterize not just the entanglement between two systems, but entanglement relative to specific spacetime descriptions.

How does Deutsch avoid these complications? Deutsch assumes a mixed state \(S_3\) of the older system prior to the interaction with the younger system. He lets it interact with an arbitrary pure state \(S_1\) younger system. After this interaction there is an entangled state \(S'\) of the two systems. Deutsch computes the mixed state \(S_2\) of the younger system which is implied by this entangled state \(S'\). His demand for consistency then is just that this mixed state \(S_2\) develops into the mixed state \(S_3\). Now it is not at all clear that this is a legitimate way to simplify the problem of time travel in quantum mechanics. But even if we grant him this simplification there is a problem: how are we to understand these mixtures?

If we take an ignorance interpretation of mixtures we run into trouble. For suppose that we assume that in each individual case each older system is either in state \(\vc{+}_3\) or in state \(\vc{-}_3\) prior to the interaction. Then we regain our paradox. Deutsch instead recommends the following, many worlds, picture of mixtures. Suppose we start with state \(\vc{+}_1\) in all worlds. In some of the many worlds the older system will be in the \(\vc{+}_3\) state, let us call them A -worlds, and in some worlds, B -worlds, it will be in the \(\vc{-}_3\) state. Thus in A -worlds after interaction we will have state \(\vc{-}_2\) , and in B -worlds we will have state \(\vc{+}_2\). During time travel the \(\vc{-}_2\) state will remain the same, i.e., turn into state \(\vc{-}_3\), but the systems in question will travel from A -worlds to B -worlds. Similarly the \(\vc{+}\) \(_2\) states will travel from the B -worlds to the A -worlds, thus preserving consistency.

Now whatever one thinks of the merits of many worlds interpretations, and of this understanding of it applied to mixtures, in the end one does not obtain genuine time travel in Deutsch’s account. The systems in question travel from one time in one world to another time in another world, but no system travels to an earlier time in the same world. (This is so at least in the normal sense of the word “world”, the sense that one means when, for instance, one says “there was, and will be, only one Elvis Presley in this world.”) Thus, even if it were a reasonable view, it is not quite as interesting as it may have initially seemed. (See Wallace 2012 for a more sympathetic treatment, that explores several further implications of accepting time travel in conjunction with the many worlds interpretation.)

We close by acknowledging that Deutsch’s starting point—the claim that this computational model captures the essential features of quantum systems in a spacetime with CTCs—has been the subject of some debate. Several physicists have pursued a quite different treatment of evolution of quantum systems through CTC’s, based on considering the “post-selected” state (see Lloyd et al. 2011). Their motivations for implementing the consistency condition in terms of the post-selected state reflects a different stance towards quantum foundations. A different line of argument aims to determine whether Deutsch’s treatment holds as an appropriate limiting case of a more rigorous treatment, such as quantum field theory in curved spacetimes. For example, Verch (2020) establishes several results challenging the assumption that Deutsch’s treatment is tied to the presence of CTC’s, or that it is compatible with the entanglement structure of quantum fields.

What remains of the grandfather paradox in general relativistic time travel worlds is the fact that in some cases the states on edgeless spacelike surfaces are “overconstrained”, so that one has less than the usual freedom in specifying conditions on such a surface, given the time-travel structure, and in some cases such states are “underconstrained”, so that states on edgeless space-like surfaces do not determine what happens elsewhere in the way that they usually do, given the time travel structure. There can also be mixtures of those two types of cases. The extent to which states are overconstrained and/or underconstrained in realistic models is as yet unclear, though it would be very surprising if neither obtained. The extant literature has primarily focused on the problem of overconstraint, since that, often, either is regarded as a metaphysical obstacle to the possibility time travel, or as an epistemological obstacle to the plausibility of time travel in our world. While it is true that our world would be quite different from the way we normally think it is if states were overconstrained, underconstraint seems at least as bizarre as overconstraint. Nonetheless, neither directly rules out the possibility of time travel.

If time travel entailed contradictions then the issue would be settled. And indeed, most of the stories employing time travel in popular culture are logically incoherent: one cannot “change” the past to be different from what it was, since the past (like the present and the future) only occurs once. But if the only requirement demanded is logical coherence, then it seems all too easy. A clever author can devise a coherent time-travel scenario in which everything happens just once and in a consistent way. This is just too cheap: logical coherence is a very weak condition, and many things we take to be metaphysically impossible are logically coherent. For example, it involves no logical contradiction to suppose that water is not molecular, but if both chemistry and Kripke are right it is a metaphysical impossibility. We have been interested not in logical possibility but in physical possibility. But even so, our conditions have been relatively weak: we have asked only whether time-travel is consistent with the universal validity of certain fundamental physical laws and with the notion that the physical state on a surface prior to the time travel region be unconstrained. It is perfectly possible that the physical laws obey this condition, but still that time travel is not metaphysically possible because of the nature of time itself. Consider an analogy. Aristotle believed that water is homoiomerous and infinitely divisible: any bit of water could be subdivided, in principle, into smaller bits of water. Aristotle’s view contains no logical contradiction. It was certainly consistent with Aristotle’s conception of water that it be homoiomerous, so this was, for him, a conceptual possibility. But if chemistry is right, Aristotle was wrong both about what water is like and what is possible for it. It can’t be infinitely divided, even though no logical or conceptual analysis would reveal that.

Similarly, even if all of our consistency conditions can be met, it does not follow that time travel is physically possible, only that some specific physical considerations cannot rule it out. The only serious proof of the possibility of time travel would be a demonstration of its actuality. For if we agree that there is no actual time travel in our universe, the supposition that there might have been involves postulating a substantial difference from actuality, a difference unlike in kind from anything we could know if firsthand. It is unclear to us exactly what the content of possible would be if one were to either maintain or deny the possibility of time travel in these circumstances, unless one merely meant that the possibility is not ruled out by some delineated set of constraints. As the example of Aristotle’s theory of water shows, conceptual and logical “possibility” do not entail possibility in a full-blooded sense. What exactly such a full-blooded sense would be in case of time travel, and whether one could have reason to believe it to obtain, remain to us obscure.

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How to cite this entry . Preview the PDF version of this entry at the Friends of the SEP Society . Look up topics and thinkers related to this entry at the Internet Philosophy Ontology Project (InPhO). Enhanced bibliography for this entry at PhilPapers , with links to its database.

• Adlam, Emily, unpublished, “ Is There Causation in Fundamental Physics? New Insights from Process Matrices and Quantum Causal Modelling ”, 2022, arXiv: 2208.02721. doi:10.48550/ARXIV.2208.02721
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causation: backward | determinism: causal | quantum mechanics | quantum mechanics: retrocausality | space and time: being and becoming in modern physics | time machines | time travel

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Is Time Travel Possible?

We all travel in time! We travel one year in time between birthdays, for example. And we are all traveling in time at approximately the same speed: 1 second per second.

We typically experience time at one second per second. Credit: NASA/JPL-Caltech

NASA's space telescopes also give us a way to look back in time. Telescopes help us see stars and galaxies that are very far away . It takes a long time for the light from faraway galaxies to reach us. So, when we look into the sky with a telescope, we are seeing what those stars and galaxies looked like a very long time ago.

However, when we think of the phrase "time travel," we are usually thinking of traveling faster than 1 second per second. That kind of time travel sounds like something you'd only see in movies or science fiction books. Could it be real? Science says yes!

This image from the Hubble Space Telescope shows galaxies that are very far away as they existed a very long time ago. Credit: NASA, ESA and R. Thompson (Univ. Arizona)

How do we know that time travel is possible?

More than 100 years ago, a famous scientist named Albert Einstein came up with an idea about how time works. He called it relativity. This theory says that time and space are linked together. Einstein also said our universe has a speed limit: nothing can travel faster than the speed of light (186,000 miles per second).

Einstein's theory of relativity says that space and time are linked together. Credit: NASA/JPL-Caltech

What does this mean for time travel? Well, according to this theory, the faster you travel, the slower you experience time. Scientists have done some experiments to show that this is true.

For example, there was an experiment that used two clocks set to the exact same time. One clock stayed on Earth, while the other flew in an airplane (going in the same direction Earth rotates).

After the airplane flew around the world, scientists compared the two clocks. The clock on the fast-moving airplane was slightly behind the clock on the ground. So, the clock on the airplane was traveling slightly slower in time than 1 second per second.

Credit: NASA/JPL-Caltech

Can we use time travel in everyday life?

We can't use a time machine to travel hundreds of years into the past or future. That kind of time travel only happens in books and movies. But the math of time travel does affect the things we use every day.

For example, we use GPS satellites to help us figure out how to get to new places. (Check out our video about how GPS satellites work .) NASA scientists also use a high-accuracy version of GPS to keep track of where satellites are in space. But did you know that GPS relies on time-travel calculations to help you get around town?

GPS satellites orbit around Earth very quickly at about 8,700 miles (14,000 kilometers) per hour. This slows down GPS satellite clocks by a small fraction of a second (similar to the airplane example above).

GPS satellites orbit around Earth at about 8,700 miles (14,000 kilometers) per hour. Credit: GPS.gov

However, the satellites are also orbiting Earth about 12,550 miles (20,200 km) above the surface. This actually speeds up GPS satellite clocks by a slighter larger fraction of a second.

Here's how: Einstein's theory also says that gravity curves space and time, causing the passage of time to slow down. High up where the satellites orbit, Earth's gravity is much weaker. This causes the clocks on GPS satellites to run faster than clocks on the ground.

The combined result is that the clocks on GPS satellites experience time at a rate slightly faster than 1 second per second. Luckily, scientists can use math to correct these differences in time.

If scientists didn't correct the GPS clocks, there would be big problems. GPS satellites wouldn't be able to correctly calculate their position or yours. The errors would add up to a few miles each day, which is a big deal. GPS maps might think your home is nowhere near where it actually is!

In Summary:

Yes, time travel is indeed a real thing. But it's not quite what you've probably seen in the movies. Under certain conditions, it is possible to experience time passing at a different rate than 1 second per second. And there are important reasons why we need to understand this real-world form of time travel.

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Paradox-Free Time Travel Is Theoretically Possible, Researchers Say

Matthew S. Schwartz

A dog dressed as Marty McFly from Back to the Future attends the Tompkins Square Halloween Dog Parade in 2015. New research says time travel might be possible without the problems McFly encountered. Timothy A. Clary/AFP via Getty Images hide caption

A dog dressed as Marty McFly from Back to the Future attends the Tompkins Square Halloween Dog Parade in 2015. New research says time travel might be possible without the problems McFly encountered.

"The past is obdurate," Stephen King wrote in his book about a man who goes back in time to prevent the Kennedy assassination. "It doesn't want to be changed."

Turns out, King might have been on to something.

Countless science fiction tales have explored the paradox of what would happen if you went back in time and did something in the past that endangered the future. Perhaps one of the most famous pop culture examples is in Back to the Future , when Marty McFly goes back in time and accidentally stops his parents from meeting, putting his own existence in jeopardy.

But maybe McFly wasn't in much danger after all. According a new paper from researchers at the University of Queensland, even if time travel were possible, the paradox couldn't actually exist.

Researchers ran the numbers and determined that even if you made a change in the past, the timeline would essentially self-correct, ensuring that whatever happened to send you back in time would still happen.

"Say you traveled in time in an attempt to stop COVID-19's patient zero from being exposed to the virus," University of Queensland scientist Fabio Costa told the university's news service .

"However, if you stopped that individual from becoming infected, that would eliminate the motivation for you to go back and stop the pandemic in the first place," said Costa, who co-authored the paper with honors undergraduate student Germain Tobar.

"This is a paradox — an inconsistency that often leads people to think that time travel cannot occur in our universe."

A variation is known as the "grandfather paradox" — in which a time traveler kills their own grandfather, in the process preventing the time traveler's birth.

The logical paradox has given researchers a headache, in part because according to Einstein's theory of general relativity, "closed timelike curves" are possible, theoretically allowing an observer to travel back in time and interact with their past self — potentially endangering their own existence.

But these researchers say that such a paradox wouldn't necessarily exist, because events would adjust themselves.

Take the coronavirus patient zero example. "You might try and stop patient zero from becoming infected, but in doing so, you would catch the virus and become patient zero, or someone else would," Tobar told the university's news service.

In other words, a time traveler could make changes, but the original outcome would still find a way to happen — maybe not the same way it happened in the first timeline but close enough so that the time traveler would still exist and would still be motivated to go back in time.

"No matter what you did, the salient events would just recalibrate around you," Tobar said.

The paper, "Reversible dynamics with closed time-like curves and freedom of choice," was published last week in the peer-reviewed journal Classical and Quantum Gravity . The findings seem consistent with another time travel study published this summer in the peer-reviewed journal Physical Review Letters. That study found that changes made in the past won't drastically alter the future.

Bestselling science fiction author Blake Crouch, who has written extensively about time travel, said the new study seems to support what certain time travel tropes have posited all along.

"The universe is deterministic and attempts to alter Past Event X are destined to be the forces which bring Past Event X into being," Crouch told NPR via email. "So the future can affect the past. Or maybe time is just an illusion. But I guess it's cool that the math checks out."

• time travel
• grandfather paradox

Yuri Gagarin: First Human in Space

On April 12, 1961, the era of human spaceflight began when the Cosmonaut Yuri Gagarin became the first human to orbit the Earth in his Vostock I spacecraft. The flight lasted 108 minutes.

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April 4, 2006 feature

Professor predicts human time travel this century

By Lisa Zyga , Phys.org

With a brilliant idea and equations based on Einstein’s relativity theories, Ronald Mallett from the University of Connecticut has devised an experiment to observe a time traveling neutron in a circulating light beam. While his team still needs funding for the project, Mallett calculates that the possibility of time travel using this method could be verified within a decade.

Black holes, wormholes, and cosmic strings - each of these phenomena has been proposed as a method for time travel, but none seem feasible, for (at least) one major reason. Although theoretically they could distort space-time, they all require an unthinkably gigantic amount of mass.

Mallett, a U Conn Physics Professor for 30 years, considered an alternative to these time travel methods based on Einstein’s famous relativity equation: E=mc 2 .

“Einstein showed that mass and energy are the same thing,” said Mallett, who published his first research on time travel in 2000 in Physics Letters . “The time machine we’ve designed uses light in the form of circulating lasers to warp or loop time instead of using massive objects.”

“Say you have a cup of coffee and a spoon,” Mallett explained to PhysOrg.com . “The coffee is empty space, and the spoon is the circulating light beam. When you stir the coffee with the spoon, the coffee - or the empty space - gets twisted. Suppose you drop a sugar cube in the coffee. If empty space were twisting, you’d be able to detect it by observing a subatomic particle moving around in the space.”

And according to Einstein, whenever you do something to space, you also affect time. Twisting space causes time to be twisted, meaning you could theoretically walk through time as you walk through space.

“As physicists, our experiments deal with subatomic particles,” said Mallett. “How soon humans will be able to time travel depends largely on the success of these experiments, which will take the better part of a decade. And depending on breakthroughs, technology, and funding, I believe that human time travel could happen this century.”

Step back a minute (sorry, only figuratively). How do we know that time is not merely a human invention, and that manipulating it just doesn’t make sense?

“What is time? That is a very, very difficult question,” said Mallett. “Time is a way of separating events from each other. Even without thinking about time, we can see that things change, seasons change, people change. The fact that the world changes is an intrinsic feature of the physical world, and time is independent of whether or not we have a name for it.

“To physicists, time is what’s measured by clocks. Using this definition, we can manipulate time by changing the rate of clocks, which changes the rate at which events occur. Einstein showed that time is affected by motion, and his theories have been demonstrated experimentally by comparing time on an atomic clock that has traveled around the earth on a jet. It’s slower than a clock on earth.”

Although the jet-flying clock regained its normal pace when it landed, it never caught up with earth clocks - which means that we have a time traveler from the past among us already, even though it thinks it’s in the future.

Some people show concern over time traveling, although Mallett - an advocate of the Parallel Universes theory - assures us that time machines will not present any danger.

“The Grandfather Paradox [where you go back in time and kill your grandfather] is not an issue,” said Mallett. “In a sense, time travel means that you’re traveling both in time and into other universes. If you go back into the past, you’ll go into another universe. As soon as you arrive at the past, you’re making a choice and there’ll be a split. Our universe will not be affected by what you do in your visit to the past.”

In light of this causal “safety,” it’s kind of ironic that what prompted Mallett as a child to investigate time travel was a desire to change the past in hopes of a different future. When he was 10 years old, his father died of a heart attack at age 33. After reading The Time Machine by H.G. Wells, Mallett was determined to find a way to go back and warn his father about the dangers of smoking.

This personal element fueled Mallett’s perseverance to study science, master Einstein’s equations, and build a professional career with many high notes. Since the ‘70s, his research has included quantum gravity, relativistic cosmology and gauge theories, and he plans to publish a popular science/memoir book this November 2006. With help from Bruce Henderson, the New York Times best-selling author, the book will be called Time Traveler: A Physicist’s Quest For The Ultimate Breakthrough .

Copyright 2010 PhysOrg.com. All rights reserved. This material may not be published, broadcast, rewritten or redistributed in whole or part without the express written permission of PhysOrg.com.

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April 1961 – first human entered space.

Yuri Gagarin from the Soviet Union was the first human in space. His vehicle, Vostok 1 circled Earth at a speed of 27,400 kilometers per hour with the flight lasting 108 minutes. Vostok’s reentry was controlled by a computer. Unlike the early U.S. human spaceflight programs, Gagarin did not land inside of capsule. Instead, he ejected from the spacecraft and landed by parachute.

Time travel: Is it possible?

Science says time travel is possible, but probably not in the way you're thinking.

Albert Einstein's theory

• General relativity and GPS
• Wormhole travel
• Alternate theories

Science fiction

Is time travel possible? Short answer: Yes, and you're doing it right now — hurtling into the future at the impressive rate of one second per second. 

You're pretty much always moving through time at the same speed, whether you're watching paint dry or wishing you had more hours to visit with a friend from out of town. 

But this isn't the kind of time travel that's captivated countless science fiction writers, or spurred a genre so extensive that Wikipedia lists over 400 titles in the category "Movies about Time Travel." In franchises like " Doctor Who ," " Star Trek ," and "Back to the Future" characters climb into some wild vehicle to blast into the past or spin into the future. Once the characters have traveled through time, they grapple with what happens if you change the past or present based on information from the future (which is where time travel stories intersect with the idea of parallel universes or alternate timelines). 

Related: The best sci-fi time machines ever

Although many people are fascinated by the idea of changing the past or seeing the future before it's due, no person has ever demonstrated the kind of back-and-forth time travel seen in science fiction or proposed a method of sending a person through significant periods of time that wouldn't destroy them on the way. And, as physicist Stephen Hawking pointed out in his book " Black Holes and Baby Universes" (Bantam, 1994), "The best evidence we have that time travel is not possible, and never will be, is that we have not been invaded by hordes of tourists from the future."

Science does support some amount of time-bending, though. For example, physicist Albert Einstein 's theory of special relativity proposes that time is an illusion that moves relative to an observer. An observer traveling near the speed of light will experience time, with all its aftereffects (boredom, aging, etc.) much more slowly than an observer at rest. That's why astronaut Scott Kelly aged ever so slightly less over the course of a year in orbit than his twin brother who stayed here on Earth. 

Related: Controversially, physicist argues that time is real

There are other scientific theories about time travel, including some weird physics that arise around wormholes , black holes and string theory . For the most part, though, time travel remains the domain of an ever-growing array of science fiction books, movies, television shows, comics, video games and more. 

Einstein developed his theory of special relativity in 1905. Along with his later expansion, the theory of general relativity , it has become one of the foundational tenets of modern physics. Special relativity describes the relationship between space and time for objects moving at constant speeds in a straight line. 

The short version of the theory is deceptively simple. First, all things are measured in relation to something else — that is to say, there is no "absolute" frame of reference. Second, the speed of light is constant. It stays the same no matter what, and no matter where it's measured from. And third, nothing can go faster than the speed of light.

From those simple tenets unfolds actual, real-life time travel. An observer traveling at high velocity will experience time at a slower rate than an observer who isn't speeding through space. 

While we don't accelerate humans to near-light-speed, we do send them swinging around the planet at 17,500 mph (28,160 km/h) aboard the International Space Station . Astronaut Scott Kelly was born after his twin brother, and fellow astronaut, Mark Kelly . Scott Kelly spent 520 days in orbit, while Mark logged 54 days in space. The difference in the speed at which they experienced time over the course of their lifetimes has actually widened the age gap between the two men.

"So, where[as] I used to be just 6 minutes older, now I am 6 minutes and 5 milliseconds older," Mark Kelly said in a panel discussion on July 12, 2020, Space.com previously reported . "Now I've got that over his head."

General relativity and GPS time travel

The difference that low earth orbit makes in an astronaut's life span may be negligible — better suited for jokes among siblings than actual life extension or visiting the distant future — but the dilation in time between people on Earth and GPS satellites flying through space does make a difference. 

Read more: Can we stop time?

The Global Positioning System , or GPS, helps us know exactly where we are by communicating with a network of a few dozen satellites positioned in a high Earth orbit. The satellites circle the planet from 12,500 miles (20,100 kilometers) away, moving at 8,700 mph (14,000 km/h). 

According to special relativity, the faster an object moves relative to another object, the slower that first object experiences time. For GPS satellites with atomic clocks, this effect cuts 7 microseconds, or 7 millionths of a second, off each day, according to the American Physical Society publication Physics Central .  

Read more: Could Star Trek's faster-than-light warp drive actually work?

Then, according to general relativity, clocks closer to the center of a large gravitational mass like Earth tick more slowly than those farther away. So, because the GPS satellites are much farther from the center of Earth compared to clocks on the surface, Physics Central added, that adds another 45 microseconds onto the GPS satellite clocks each day. Combined with the negative 7 microseconds from the special relativity calculation, the net result is an added 38 microseconds. 

This means that in order to maintain the accuracy needed to pinpoint your car or phone — or, since the system is run by the U.S. Department of Defense, a military drone — engineers must account for an extra 38 microseconds in each satellite's day. The atomic clocks onboard don’t tick over to the next day until they have run 38 microseconds longer than comparable clocks on Earth.

Given those numbers, it would take more than seven years for the atomic clock in a GPS satellite to un-sync itself from an Earth clock by more than a blink of an eye. (We did the math: If you estimate a blink to last at least 100,000 microseconds, as the Harvard Database of Useful Biological Numbers does, it would take thousands of days for those 38 microsecond shifts to add up.) 

This kind of time travel may seem as negligible as the Kelly brothers' age gap, but given the hyper-accuracy of modern GPS technology, it actually does matter. If it can communicate with the satellites whizzing overhead, your phone can nail down your location in space and time with incredible accuracy. 

Can wormholes take us back in time?

General relativity might also provide scenarios that could allow travelers to go back in time, according to NASA . But the physical reality of those time-travel methods is no piece of cake. 

Wormholes are theoretical "tunnels" through the fabric of space-time that could connect different moments or locations in reality to others. Also known as Einstein-Rosen bridges or white holes, as opposed to black holes, speculation about wormholes abounds. But despite taking up a lot of space (or space-time) in science fiction, no wormholes of any kind have been identified in real life. 

Related: Best time travel movies

"The whole thing is very hypothetical at this point," Stephen Hsu, a professor of theoretical physics at the University of Oregon, told Space.com sister site Live Science . "No one thinks we're going to find a wormhole anytime soon."

Primordial wormholes are predicted to be just 10^-34 inches (10^-33 centimeters) at the tunnel's "mouth". Previously, they were expected to be too unstable for anything to be able to travel through them. However, a study claims that this is not the case, Live Science reported . 

The theory, which suggests that wormholes could work as viable space-time shortcuts, was described by physicist Pascal Koiran. As part of the study, Koiran used the Eddington-Finkelstein metric, as opposed to the Schwarzschild metric which has been used in the majority of previous analyses.

In the past, the path of a particle could not be traced through a hypothetical wormhole. However, using the Eddington-Finkelstein metric, the physicist was able to achieve just that.

Koiran's paper was described in October 2021, in the preprint database arXiv , before being published in the Journal of Modern Physics D.

Alternate time travel theories

While Einstein's theories appear to make time travel difficult, some researchers have proposed other solutions that could allow jumps back and forth in time. These alternate theories share one major flaw: As far as scientists can tell, there's no way a person could survive the kind of gravitational pulling and pushing that each solution requires.

Infinite cylinder theory

Astronomer Frank Tipler proposed a mechanism (sometimes known as a Tipler Cylinder ) where one could take matter that is 10 times the sun's mass, then roll it into a very long, but very dense cylinder. The Anderson Institute , a time travel research organization, described the cylinder as "a black hole that has passed through a spaghetti factory."

After spinning this black hole spaghetti a few billion revolutions per minute, a spaceship nearby — following a very precise spiral around the cylinder — could travel backward in time on a "closed, time-like curve," according to the Anderson Institute. 

The major problem is that in order for the Tipler Cylinder to become reality, the cylinder would need to be infinitely long or be made of some unknown kind of matter. At least for the foreseeable future, endless interstellar pasta is beyond our reach.

Time donuts

Theoretical physicist Amos Ori at the Technion-Israel Institute of Technology in Haifa, Israel, proposed a model for a time machine made out of curved space-time — a donut-shaped vacuum surrounded by a sphere of normal matter.

"The machine is space-time itself," Ori told Live Science . "If we were to create an area with a warp like this in space that would enable time lines to close on themselves, it might enable future generations to return to visit our time."

Amos Ori is a theoretical physicist at the Technion-Israel Institute of Technology in Haifa, Israel. His research interests and publications span the fields of general relativity, black holes, gravitational waves and closed time lines.

There are a few caveats to Ori's time machine. First, visitors to the past wouldn't be able to travel to times earlier than the invention and construction of the time donut. Second, and more importantly, the invention and construction of this machine would depend on our ability to manipulate gravitational fields at will — a feat that may be theoretically possible but is certainly beyond our immediate reach.

Time travel has long occupied a significant place in fiction. Since as early as the "Mahabharata," an ancient Sanskrit epic poem compiled around 400 B.C., humans have dreamed of warping time, Lisa Yaszek, a professor of science fiction studies at the Georgia Institute of Technology in Atlanta, told Live Science .  

Every work of time-travel fiction creates its own version of space-time, glossing over one or more scientific hurdles and paradoxes to achieve its plot requirements. 

Some make a nod to research and physics, like " Interstellar ," a 2014 film directed by Christopher Nolan. In the movie, a character played by Matthew McConaughey spends a few hours on a planet orbiting a supermassive black hole, but because of time dilation, observers on Earth experience those hours as a matter of decades. 

Others take a more whimsical approach, like the "Doctor Who" television series. The series features the Doctor, an extraterrestrial "Time Lord" who travels in a spaceship resembling a blue British police box. "People assume," the Doctor explained in the show, "that time is a strict progression from cause to effect, but actually from a non-linear, non-subjective viewpoint, it's more like a big ball of wibbly-wobbly, timey-wimey stuff." 

Long-standing franchises like the "Star Trek" movies and television series, as well as comic universes like DC and Marvel Comics, revisit the idea of time travel over and over. 

Related: Marvel movies in order: chronological & release order

Here is an incomplete (and deeply subjective) list of some influential or notable works of time travel fiction:

Books about time travel:

• Rip Van Winkle (Cornelius S. Van Winkle, 1819) by Washington Irving
• A Christmas Carol (Chapman & Hall, 1843) by Charles Dickens
• The Time Machine (William Heinemann, 1895) by H. G. Wells
• A Connecticut Yankee in King Arthur's Court (Charles L. Webster and Co., 1889) by Mark Twain
• The Restaurant at the End of the Universe (Pan Books, 1980) by Douglas Adams
• A Tale of Time City (Methuen, 1987) by Diana Wynn Jones
• The Outlander series (Delacorte Press, 1991-present) by Diana Gabaldon
• Harry Potter and the Prisoner of Azkaban (Bloomsbury/Scholastic, 1999) by J. K. Rowling
• Thief of Time (Doubleday, 2001) by Terry Pratchett
• The Time Traveler's Wife (MacAdam/Cage, 2003) by Audrey Niffenegger
• All You Need is Kill (Shueisha, 2004) by Hiroshi Sakurazaka

Movies about time travel:

• Planet of the Apes (1968)
• Superman (1978)
• Time Bandits (1981)
• The Terminator (1984)
• Back to the Future series (1985, 1989, 1990)
• Star Trek IV: The Voyage Home (1986)
• Bill & Ted's Excellent Adventure (1989)
• Groundhog Day (1993)
• Galaxy Quest (1999)
• The Butterfly Effect (2004)
• 13 Going on 30 (2004)
• The Lake House (2006)
• Meet the Robinsons (2007)
• Hot Tub Time Machine (2010)
• Midnight in Paris (2011)
• Looper (2012)
• X-Men: Days of Future Past (2014)
• Edge of Tomorrow (2014)
• Interstellar (2014)
• Doctor Strange (2016)
• A Wrinkle in Time (2018)
• The Last Sharknado: It's About Time (2018)
• Avengers: Endgame (2019)
• Tenet (2020)
• Palm Springs (2020)
• Zach Snyder's Justice League (2021)
• The Tomorrow War (2021)

Television about time travel:

• Doctor Who (1963-present)
• The Twilight Zone (1959-1964) (multiple episodes)
• Star Trek (multiple series, multiple episodes)
• Samurai Jack (2001-2004)
• Lost (2004-2010)
• Phil of the Future (2004-2006)
• Steins;Gate (2011)
• Outlander (2014-2023)
• Loki (2021-present)

Games about time travel:

• Chrono Trigger (1995)
• TimeSplitters (2000-2005)
• Kingdom Hearts (2002-2019)
• Prince of Persia: Sands of Time (2003)
• God of War II (2007)
• Ratchet and Clank Future: A Crack In Time (2009)
• Sly Cooper: Thieves in Time (2013)
• Dishonored 2 (2016)
• Titanfall 2 (2016)
• Outer Wilds (2019)

Additional resources

Explore physicist Peter Millington's thoughts about Stephen Hawking's time travel theories at The Conversation . Check out a kid-friendly explanation of real-world time travel from NASA's Space Place . For an overview of time travel in fiction and the collective consciousness, read " Time Travel: A History " (Pantheon, 2016) by James Gleik. 

Join our Space Forums to keep talking space on the latest missions, night sky and more! And if you have a news tip, correction or comment, let us know at: [email protected].

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Ailsa is a staff writer for How It Works magazine, where she writes science, technology, space, history and environment features. Based in the U.K., she graduated from the University of Stirling with a BA (Hons) journalism degree. Previously, Ailsa has written for Cardiff Times magazine, Psychology Now and numerous science bookazines. 

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World's top 10 iconic landmarks

TRAVEL TRENDS , WORLD Created : May 9, 2024, 00:00 IST

Across the globe, there are certain landmarks that have been capturing the imagination, and evoking a sense of awe for the longest time. Apart from the ancient wonders, there are also some modern marvels that serve as the perfect example of human ingenuity, cultural heritage, and architectural brilliance. So, here are top 10 must-visit landmarks in the world:

Eiffel Tower, Paris

A symbol of romance and elegance, the Eiffel Tower is one of the most recognizable landmarks in the world. Designed by Gustave Eiffel for the 1889 World's Fair, this wrought-iron lattice tower offers breathtaking views of Paris from its observation decks.

Great Wall of China

Spanning over 13,000 miles across northern China, the Great Wall is an architectural marvel that dates back over 2,000 years. Built to protect the Chinese Empire from invasions, this ancient fortification is a UNESCO World Heritage Site and a testament to the ingenuity and perseverance of the Chinese people.

Machu Picchu, Peru

Nestled high in the Andes Mountains, Machu Picchu is an ancient Incan citadel that remains one of the most mysterious and captivating archaeological sites in the world. Built in the 15th century and abandoned during the Spanish conquest, this UNESCO World Heritage Site offers visitors a glimpse into the rich history and culture of the Inca civilization.

Taj Mahal, India

Regarded as one of the most beautiful buildings ever created, the Taj Mahal is a masterpiece of Mughal architecture and a symbol of enduring love. Built by Emperor Shah Jahan in memory of his beloved wife Mumtaz Mahal, this white marble mausoleum is a UNESCO World Heritage Site and one of the New Seven Wonders of the World.

Sydney Opera House, Australia

A symbol of modern architecture and artistic excellence, the Sydney Opera House is an iconic landmark that graces the shores of Sydney Harbour. Designed by Danish architect Jørn Utzon, this multi-venue performing arts center hosts over 1,500 performances annually and attracts millions of visitors from around the world.

Statue of Liberty, New York City

A universal symbol of freedom and democracy, the Statue of Liberty stands proudly on Liberty Island in New York Harbor. This colossal copper statue depicts Libertas, the Roman goddess of freedom, and welcomes immigrants and visitors to the land of opportunity.

Colosseum, Rome

An iconic symbol of ancient Rome, the Colosseum is the largest amphitheater ever built and a UNESCO World Heritage Site. This monumental structure hosted gladiatorial contests, animal hunts, and other public spectacles, attracting crowds of up to 80,000 spectators.

Petra, Jordan

Carved into the rose-red cliffs of southern Jordan, Petra is an ancient Nabatean city that flourished as a trading hub in the first century BC. Known as the ‘Rose City’ for its stunning rock-cut architecture and pink sandstone cliffs, Petra is a UNESCO World Heritage Site and one of the New Seven Wonders of the World.

The Pyramids of Giza, Egypt

Built over 4,500 years ago, the Pyramids of Giza are the last surviving wonders of the ancient world. These majestic structures, including the Great Pyramid of Khufu, the Pyramid of Khafre, and the Pyramid of Menkaure, stand as enduring symbols of Egypt's pharaonic past and architectural prowess.

Burj Khalifa, Dubai

Soaring over the skyline of Dubai, the Burj Khalifa is indeed a feat of modern engineering. Standing at over 828 m tall, this iconic skyscraper offers panoramic views of the city from its observation decks and is a symbol of Dubai's ambition and innovation.

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Across the globe, there are certain landmarks that have been capturing the imagination, and evoking a sense of awe for the longest time. Apart from the ancient wonders, there are also some modern marv...

BREAKING: Rep. Marjorie Taylor Greene is forcing a vote to oust Speaker Johnson over Ukraine aid

Boeing forced to scrub first crewed Starliner launch to the space station

NASA and Boeing were forced to stand down from an attempted launch to the International Space Station on Monday because of a last-minute issue that cropped up with a valve on the spacecraft’s rocket.

Boeing’s Starliner capsule had been scheduled to lift off at 10:34 p.m. ET from Florida’s Cape Canaveral Space Force Station on its first crewed test flight. NASA astronauts Barry “Butch” Wilmore and Sunita Williams were on board the capsule and strapped into their seats when the launch attempt was called off, roughly two hours ahead of the planned liftoff.

NASA announced early Tuesday that a second attempt would occur no earlier than Friday .

Mission controllers declared Monday’s launch “scrub” after an anomaly was detected on a valve on United Launch Alliance’s Atlas V rocket, which the Starliner capsule was to ride into orbit.

United Launch Alliance officials said in a post on X that the launch attempt was scrapped “out of an abundance of caution for the safety of the flight and pad crew,” adding that the team needs “additional time to complete a full assessment.”

The analysis will include whether the pressure regulation valve, located on the rocket’s upper stage, needs to be replaced, which may cause a longer delay.

The crewed Starliner flight, when it occurs, will be a crucial final test before NASA can authorize Boeing to conduct routine flights to and from the space station.

Officials at NASA and Boeing have said safety is paramount for the spacecraft’s first flight with humans onboard.

The scrubbed launch is yet another setback for Boeing, which has already dealt with years of delays and budget overruns with its Starliner program. It has fallen significantly behind SpaceX, which has been flying crewed missions to and from the space station for NASA since 2020.

Both SpaceX’s Crew Dragon capsule and Boeing’s Starliner craft were developed as part of NASA’s Commercial Crew Program. The initiative began more than a decade ago, following the retirement of the agency’s space shuttles, to support private companies in building new space vehicles to take astronauts to low-Earth orbit.

Starliner’s first uncrewed flight in 2019 was thwarted by software issues, forcing mission controllers to cut the test short before the vehicle could attempt to rendezvous and dock with the ISS. A second attempt was then delayed several times by fuel valve issues, and it wasn’t until 2022 that Boeing was able to carry out a successful uncrewed flight to and from the space station .

Denise Chow is a reporter for NBC News Science focused on general science and climate change.

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The NFL schedule release is upon us.

This has become like a holiday for NFL fans to see the roadmap their teams will travel each season. For the Jets, this year’s release is a little less exciting than last year, when the anticipation was the Jets would be relevant to the TV networks for the first time in years with Aaron Rodgers at quarterback.

The Jets were picked for five prime-time games last season, the most a team can be chosen for. It will be interesting to see whether Rodgers and the Jets still carry the same cachet this year with the networks and land in prime time again.

For this week’s newsletter, I will try to design the perfect schedule from a Jets perspective.

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Israel-Gaza latest: Israeli officials 'deeply frustrated' over US move to halt arms shipment - as IDF launches another Rafah operation

A senior Biden administration official says the US paused a shipment of bombs to Israel last week over fears of a full-scale assault on Rafah. Listen to a Daily podcast on why a ceasefire hasn't happened while you scroll.

Wednesday 8 May 2024 22:25, UK

• Israel-Hamas war
• US paused shipment of bombs to Israel over fears of Rafah assault
• Israeli officials 'deeply frustrated' over US move to halt arms shipment
• Mark Stone analysis:  Biden's red line is all very well, as long as he takes action if it's crossed
• Israel launches another Rafah operation
• No ceasefire if Rafah operation continues, Hamas tells Israel
• Alistair Bunkall analysis : Final roll of dice for Netanyahu as he gambles on Rafah
• Israel says it has reopened Kerem Shalom border crossing
• Listen to the Daily above and tap here to follow wherever you get your podcasts
• Live updates by Lauren Russell and (earlier)  Bhvishya Patel

Children who have been injured during the war in Gaza face a "lifetime of surgery", a reconstructive doctor has said. 

Dr Ghassan Abu Sitta a plastic and reconstructive surgeon told Sky News at least half of the approximate 75,000 Palestinian's wounded in Gaza due to Israeli strikes are children, a lot of them with very severe injuries. 

"[A lot of them] will need complex surgeries," Dr Abu Sitta said. 

"What we know is that in the long-term children with war injuries will need between eight and 12 surgeries before they reach adult age.

"There is a lifetime of surgery awaiting these children."

Earlier this week, Unicef warned that 600,000 children are currently sheltering in the Gaza city of Rafah.

The charity warned of further catastrophe for children if Israel launches a military ground operation.

"Rafah is now a city of children, who have nowhere safe to go in Gaza," Unicef said, adding that the physical and mental states of the children are already weakened.

Dr Victoria Rose, a consultant plastic surgeon who has recently come back from Gaza, said the majority of people she treated were children.

"The children are basically running around on the streets outside hospitals, so they get hit by shrapnel after bombs go off," she said. 

"It was just non-stop... shrapnel wounds, blast wounds and burns."

Throughout the day, we've been reporting on what the Israel Defence Forces says is a "precise counterterrorism operation in specific areas of eastern Rafah".

Troops are also "conducting targeted raids on the Gazan side of the Rafah crossing", it has said. 

In the video below, our military analyst Michael Clarke  explains the situation in Gaza, including what the IDF could do next and if its recent movements near the Rafah crossing represent a long-awaited "strike" in the besieged area.

The UN health agency only has enough fuel to run its medical operations in southern Gaza for the next three days, the head of the World Health Organisation has warned. 

The closure of the Rafah crossing between Egypt and Gaza has exacerbated an already dire aid situation, and Tedros Adhanom Ghebreyesus said it has meant no fuel has been able to make it into the territory. 

One of the hospitals in Rafah has already had to close, making it hard to sustain "lifesaving support," he added. 

A lack of fuel has also meant WHO has suspended its operations in the north of Gaza "for now and for the coming week," Dr Rik Peeperkorn, the agency's representative to the occupied Palestinian territory, said.

Minimal medical equipment is still being provided to the region, he said, but warned it was not enough. 

"It's what I call a band-aid," Dr Peeperkorn said. "It will not help avoid substantial additional mortality and morbidity [if] there is a full-scale military operation."

 Pro-Palestinian protesters have clashed with police after erecting barricades to block one of the entrances to the University of Amsterdam.

Those involved in the protest vowed to stay put until the institution severs all ties with Israel.

The protests mirror those that are currently ongoing at Cambridge and Oxford in the UK and those in the US that led to around 2,500 arrests.

The University of Amsterdam said it decided to call the police due to concerns about security. Local residents also reported feeling unsafe. 

It added that classes at one of the universities oldest buildings - the Oudemanhuispoort - could not continue due to the protest.

The university urged all those protesting to leave and said it continues to meet with demonstrators in order to try to reach an agreement.

An Israeli soldier has been "lightly injured" after a number of launches were fired in the area of Kerem Shalom, the Israel Defence Forces has said. 

In an update posted on Telegram, the military said the launches came from Rafah in southern Gaza. 

It blamed Hamas for the attacks, saying the militant group continues to "deliberately endanger" civilians. 

"Moreover, the terrorist organization continues to fire launches from populated zones in the area of Rafah toward the Kerem Shalom Crossing to attack IDF troops, as well as the functioning of the crossing," it added. 

This morning, Israel said it reopened the Kerem Shalom border crossing to allow aid into Gaza. 

It claimed it had been shut for security reasons following an attack. 

However, the UN has said aid deliveries have failed to restart. 

By Mark Stone , US correspondent 

Setting red lines is all very well, as long as you follow through when they are crossed. 

President Joe Biden knows that all too well. 

Too often these lines in the sand turn out to be a flawed tool of geopolitical diplomacy. 

Western leaders throw them down in key-note speeches as unequivocal threats. "Cross the line, if you dare…" is the rhetoric.

Barack Obama's chemical weapons red line with Syria's Assad in 2012 was crossed. 

Biden's Ukraine red line with Putin in 2021 was crossed.

Every red line is distinct and, of course, they vary in terms of their gravity of the event they are seeking to prevent. 

But the principle behind laying them is the same, as is the message set when they are crossed. 

Rafah has become Biden's red line

Over the past six months, as Israel has sought to defeat Hamas in Gaza, President Biden didn't think he'd need to lay out red lines. 

After all, Israel is one of America's closest allies. 

Instead, the Biden Administration thought gentle diplomacy and frank back-channels with a 'close friend of America' would do the trick. 

But gradually, as Biden and the Netanyahu government increasingly diverged on protecting civilians and a plan for 'the day after' in Gaza, a red line began to appear - Rafah. 

This has become Biden's red line for Israel. 

The American president has repeatedly made clear his opposition to Netanyahu's insistence on a ground invasion of the southern Gazan city (Netanyahu's own red line) where about 1.4 million people are living, half of them under 18. 

The Israeli military has not (yet) moved into Rafah city but is instead concentrating its operations to the east of the city and around the crossing to Egypt. 

That fact has allowed the Biden administration to claim that its red line hasn’t yet been crossed. 

"They didn't describe it as a major ground operation," spokesman John Kirby said this week. 

Is the halting of aid just symbolic?

Sometimes, red lines are smashed through. Sometimes, they are gradually chipped away at. 

The announcement now, leaked several days ago, that America has "paused" a shipment of weapons to Israel is significant. 

It's not been done before and symbolically for Israel, in the middle of its longest and most critical war, it looks terrible. 

It's an attempt by Biden to plug a hole in his red line. A warning to Netanyahu. 

Israeli officials are said to be "deeply frustrated". That's Biden's intention.

But really, is it just symbolic? 

It's pretty inconceivable that America would abandon Israel in terms of weapons supplies. 

This shipment may have been paused. Others will continue including, critically, defensive weapons.

Biden is pulling plenty of levers he has to influence Netanyahu. He has others like properly gripping the settler violence issue in the West Bank. 

But with every lever he pulls, there is a domestic political calculus. 

Pretty much all Republicans are against every lever he pulls and so too are a significant number of his own Democrats. 

But critical voters in key states are very pro-Palestine. It's push-me-pull-you and the election is six months away.

The conflict in Gaza is like a "game of whack-a-mole", a former Middle East adviser at the US defence department has said. 

Speaking to Sky News, Jasmine El Gamal, said there was a reason Hamas has never been defeated militarily and Israeli Prime Minister Benjamin Netanyahu needs to realise this.

"[Hamas] recruits keep increasing the more the Israelis go in," Ms El Gamal said.

"We are already seeing Hamas reconstitute in other places in Gaza, so this has really been a game of whack-a-mole than a long-term sustainable solution to the idea of weakening Hamas."

It comes as the head of the US Central Intelligence Agency is expected to travel to Israel today, where he will meet with Mr Netanyahu. 

Bill Burns is also set to meet other top officials, according to a source familiar with his travel plans. 

When asked how significant the visit is, Ms El Gamal said "very". 

"Bill Burns has close relationships with his counterparts in Israel,"  she said.

She added that Mr Burns will likely tell Mr Netanyahu that there is "no military solution" and the only way Hamas will be permanently destroyed is through a negotiation of a two-state solution. 

The Israel Defence Forces has claimed to have killed the commander of Hamas' naval unit. 

It said Ahmed Ali was targeted during strikes on Gaza City in a joint operation by the IDF and the Israel Securities Authority.

It said Ali was responsible for "several attacks on Israeli territory" and troops on the ground.

Senior Israeli officials have expressed "deep frustration" with the US over its decision to pause a shipment of ammunition going to Israel, according to a new report.

The two unidentified officials were cited by Axios as claiming the decision could even jeopardise hostage negotiations. 

It comes after Israel's UN ambassador, Gilad Erdan, said the hold-up of weapons was "very disappointing", even frustrating, during an interview with Israel's Channel 12.

Mr Erdan said: "President Joe Biden can't say he is our partner in the goal to destroy Hamas while on the other hand delay the means meant to destroy Hamas."

The US decision to halt the shipment - thought to contain around 1,800 2,000-pound (900kg) bombs and 1,700 500-pound (225kg) bombs - was made after it was said to have become concerned the weapons would be used to imminently attack the southern Gaza city of Rafah. 

US Defence Secretary Lloyd Austin later said no decisions had been made over what to do next with the halted shipment. 

He said the US was also reviewing other shipments with the potential to pause more that were due to be sent to Israel.

People in the Gaza city of Rafah have urged the international community to act and try to stop any potential offensive from Israel. 

Amjad al Shawa, director of the Palestinian NGOs Network, said he had "serious concerns" about a potential land invasion and Israeli troops controlling the Rafah border crossing.

"We are calling urgently on the international community to act in order to stop such a military incursion and to open all the crossings for the passengers and for different humanitarian and commercial items," he told charity ActionAid.

Riham Jafari, who works for ActionAid in Gaza said military action had already had a "devastating impact" on residents and the number of deaths and injuries was rising. 

"We call on the Israeli authorities to abandon this catastrophic plan and demand that all states do everything in their power to prevent a military assault in Rafah," he said.

It comes after the US defence secretary confirmed that a shipment of ammunitions headed for Israel was stopped over concern it would be used in a Rafah offensive. 

Lloyd Austin said Washington had been "very clear" on its stance over action in the city, to which Israel said it had "nothing to add". 

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Traveling Amtrak For The First Time? Here Are 14 Things To Know About Amtrak Trains

F eel the need to go outside New York City? Confused about which ride to select? Forget going to out-of-the-way airports or using crowded taxi services. The Amtrak train offers the ultimate solution to meet everyone's travel needs. Meet the luxurious train that makes passengers feel like they're at home. The endless amenities and friendly, respectful staff also make one's trip memorable.

Many Amtrak trains successfully run across hundreds of tracks in the United States (most Amtrak trains offer incredibly beautiful routes , too), and there's also a rich history behind the company. Multiple packages are available for every type of passenger, depending on budget, accommodation choice, and other factors. For a pleasant change, the Amtrak company charges no extra fees, i.e., passengers get what they see, unlike some airliners who keep adding extra surcharges to a customer's expense list till the last moment.

Related: Empire Builder: What Makes This Amtrak Route One Of The Most Scenic In The U.S.

Furthermore, passengers get extra legroom for a long journey compared to a car or air travel. The Amtrak also saves on fuel and protects one's personal vehicle from wear and tear if one uses it to travel. So, with that in mind, let's skim through some important things travelers should know about the trains, such as Amtrak train routes and prices, etc.

UPDATE: 2023/06/06 21:47 EST BY REENA JAIN

More Things To Know About Amtrak Trains For First Time Travelers

Everyone should be aware of a few things before embarking on their first Amtrak journey, regardless of the destination or length of the journey. This list has been updated with additional important information, including train routes, prices, schedules, destinations, and more to help travelers have a stress-free, relaxing, and enjoyable Amtrak journey.

The Amtrak App

The Amtrak app makes it simple for travelers to plan and reserve their trips, check Amtrak schedules and destinations, and receive real-time alerts for delays or changes. Users can access their e-tickets through the app, which provides information on Amtrak train routes and prices. Additionally, the app includes interactive maps that help passengers track the whereabouts of the train and get knowledge of the schedule. The Amtrak app has a user-friendly interface and convenient features, making train travel more simple, reliable, and enjoyable.

Expect Train Delays

Amtrak trains need to stop at some places to let large freight trains pass because they share the same tracks. It can result in unforeseen delays in travel times, especially on longer, cross-country routes. For instance, the California Zephyr from Chicago to Denver may need to stop before the Moffat Tunnel to allow a freight train to pass, delaying Amtrak schedules. Therefore, researching Amtrak schedules and destinations is beneficial before making travel plans.

Making A Pre-Departure Checklist Is Helpful

Passengers will require a number of items, including those necessary and desirable for a stress-free and memorable Amtrak trip. Some of these—like an ID—are necessary, while others are worthwhile for an enjoyable Amtrak journey.

Identification

Passengers must have a photo ID or passport with them.

Train Tickets

Passengers should have hard copies of their tickets or electronic copies of their tickets on hand.

Travel insurance

It's essential to protect oneself from unanticipated travel mishaps.

Passengers who will need to take any medication while traveling must bring it with them.

Other Essentials

Personal care items, snacks, beverages, books, and some forms of entertainment are desirable (but not necessary) for a comfortable trip, particularly on long Amtrak routes.

Stations Have Different Stopover Times

The train makes scheduled stops at various stations during a trip, and the stopover time varies depending on the type of station. If a station is a rest stop, the stopover time is comparatively longer than that of a passenger drop-off and pick-up location. Knowing if a specific stop is a designated rest stop where passengers are permitted to disembark is preferable for those who want to smoke because smoking is not permitted on the train or simply want to move around the station.

Reasonable Ticket Prices

Amtrak tickets are reasonably priced! The average price of a ticket from New York to Atlantic City is about $94 (although Amtrak's famous roomettes typically cost much more ). It is pretty budget-friendly compared to any airline fare on the same route, and it is not even the lowest price out there. The cheapest tickets on this Amtrak route can even be found for only $82.

The best thing is to avoid rush hour and book your Amtrak tickets well in advance. Plus, a cool tip: Amtrak offers saver fares on each route, where passengers can save up to 20% compared to the standard Coach fare when booked at least 7 days in advance. Furthermore, Amtrak fares for seniors (aged 65 and up) are 10% less than the standard fare on most trains in the United States, and seniors (aged 60 and up) pay 10% less for cross-border services operated by Amtrak and VIA Rail Canada.

Plenty Dining Choices

Amtrak offers many dining choices to its customers. There is something tasty and appetizing for every passenger, from adults to kids. Some of these main options include:

Passengers can access the café, which is open to passengers from all classes of the train from the early morning until late at night. Whether business or economy class, there are snacks, drinks, and food items for everyone.

• Traditional Dining

Traditional dining service for passengers in private rooms is provided in the dining car as a complimentary exclusive offering.

• Flexible Dining

Exclusive offer to First Class passengers to eat at flexible timings. Special diet menus are also available at Amtrak for customers with specific dietary requirements. The food menu comes with a detailed calorie breakdown as well.

A Variety Of Accommodations

The Amtrak from New York to Atlantic City offers various services and accommodations. Among the expert tips for first-time Amtrak passengers is to check the various room options available in order to best fit the needs and budget of a passenger.

• Seating Options

Even the Coach class has the best comfy seats. Imagine reclining sofa-like seats with spacious areas. The addition of multiple amenities combines to make the trip a relaxing one! For passengers with mobility impairments, Amtrak provides accessible seating arrangements.

• Private Rooms

Amtrak provides a unique luxurious experience to its passengers by offering private rooms to First Class. If one wants privacy and their own space on a short trip with added comfort, then a private room booking is the perfect step.

Related: California Zephyr Vs. Southwest Chief: Which Amtrak Train Route Is More Scenic?

Wi-Fi On Board

The Amtrak train comes with complimentary Wi-Fi during traveling. The speed of Amtrak Wi-Fi is less than a passenger’s home or work Wi-Fi network. Furthermore, the same Wi-Fi gets shared by all the passengers on the Amtrak from New York to Atlantic City. So, each passenger should try to surf the internet in such a limited way as not to overburden the network for the other onboard users.

The key is not downloading large files or streaming heavy videos online and keeping fellow passengers in mind while using the shared Wi-Fi. That is why limited access is provided by Amtrak so that more onboard bandwidth is available to all passengers. Access to websites with objectionable content is also restricted in this shared onboard Wi-Fi.

Onboard Upgrades

The trains are divided into classes: Coach, Business, and First Class. All these classes differ from each other, with a different set of rates and amenities for each of them. So, if space is available as unoccupied in a class, one can upgrade from one's original class to that higher class seating. It can be from Coach to Business or Business to First Class.

This smart feature can change the outlook of one's whole journey. Passengers only need to speak to the conductor onboard. They might inform about the availability of such an upgrade and let passengers purchase a new class seat.

Passengers Can Bring A Bicycle Along

Amtrak offers its customers the opportunity to bring their bicycles along. One can explore the stops along the way during a train journey - and what better way to do that than by bike? Ride the rails on board the train with a bike - sounds awesome! Amtrak offers several different services to transport a customer’s bike onboard.

Remember, though, that each train has different equipment and loading procedures that decide what service will be offered. The starting and ending destination stations also determine how or what is allowed by Amtrak regarding one's bicycle. For carry-on/train side, bicycles up to 50 lbs are allowed. Here, standard bicycle sizes apply with a maximum tire width of 2 inches.

Enjoy Guest Rewards

Amtrak offers a guest rewards system to its customers . The points earned can be used to travel on trains, stay at hotels, and shop.

• Earning Points

Customers can earn guest rewards by earning points through various means, both on and off the train. Passengers can earn 2 points for every $1 spent.

• Redeeming the Points

Customer of Amtrak can easily redeem their earned points. The reward travel begins at just 800 points, with train travel allowed to over 500 destinations.

• Member Benefits

Becoming an Amtrak member rewards customers for every ride on and off the train.

• Bonus Points

One can earn 20,000 bonus points with the new Amtrak Guest Rewards® Preferred Mastercard®. Customers can even choose from over 350,000 hotels worldwide to stay at using their collected points.

Passengers Can Bring Pets

Pets are allowed to travel on Amtrak. However, like with air travel, there are restrictions. Dogs and cats can come on trips that can take up to seven hours on most Amtrak routes in a pet carrier - of course, for a small fee. The pet carrier must also be spacious and highly ventilated, allowing for free movement of the animal, and the weight of the pet and carrier combined must be under 20 lbs.

Things to Note About Pets on Amtrak Trains

• Service animals are readily welcomed on all Amtrak services as they are not included in the pet category.
• Pets are not allowed to travel as baggage. A pet must travel with its human counterpart on a train.
• Only dogs and cats are allowed to travel as pets on Amtrak, and pets must be at least eight weeks old and up to date on all vaccinations.
• Amtrak highly recommends making pet reservations in advance of one's trip.

Related: Roomette Vs. Sleeper Bedroom: Knowing The Difference On An Amtrak

Baggage Limits

With Amtrak, customers can shed their baggage worries as the company handles everything meticulously. As baggage, a customer may bring 2 carry-on items. Here, limitations apply regarding weight and size. So, browse them beforehand. Checking this guide on what to pack for an Amtrak train ride may also be handy in terms of prioritizing what to bring (and what not to bring to save baggage weight and space).

Similarly, for checked luggage, each traveler can bring 4 bags, 2 are registered as free, and the other 2 are at $20 per bag. Again, size/weight limitations apply. For special items of baggage that require special handling or are outside the normal baggage category, Amtrak may ask for additional packing requirements and service fees. Amtrak also prohibits certain items onboard trains for security purposes.

Going with unreserved seating on a packed train? No worries. Travelers can find a Red Cap (baggage porter) to escort them to the train. They can access the track before the masses and carry their bags. Red Cap services are free, but a generous tip is recommended.

No Security Line No Waiting

Amtrak lets passengers simply go to the track and get on the train. There is no security line blockage as there is at the airport. With a reduction in airport TSA staff and more passengers flying, travelers need to get to the airport about two hours before their flight just to have enough time to check in or check a bag and clear security.

With Amtrak travel, one's ticket might be scanned before entering the platform or scanned onboard, but there is no waiting line. On the other hand, Amtrak recommends arriving 30 minutes before one's train departs.